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"Exposure" vs "Digitization

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Anonymous
August 7, 2005 4:13:51 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

After spending some time under the hood, so to speak, of RAW capture and
data, I find it increasingly difficult to use the term "exposure" to
refer to the relative degree of photon saturation in a JPEG or RAW at a
given ISO. The analog to slide film exposure is actually the analog
exposure on the sensor; the ISO settings of the digital camera are like
setting different ranges of exposure in a slide to be digitized by a
scanner.

Why then, do we call utilizing the specified range "exposure". I often
substitute the word "digitized" in this context, but it draws strange
reactions from some people.
--

<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
John P Sheehy <JPS@no.komm>
><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><

More about : exposure digitization

Anonymous
August 7, 2005 4:13:52 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

JPS@no.komm wrote:

>>> After spending some time under the hood, so to speak, of RAW capture and
>>> data, I find it increasingly difficult to use the term "exposure" to
>>> refer to the relative degree of photon saturation in a JPEG or RAW at a
>>> given ISO.
>
> If someone decides that "ISO 100 gives the best quality" and gets an
> image that utilizes only 1/16th of the RAW values available, they would
> have had a much better image if they had the camera set to ISO 1600 with
> the same aperture and shutter speed. I have a hard time saying that
> they "under-exposed" the image; it makes more sense to say that they
> under-digitized it (quantized it) by using too low of an ISO.

Is or is not the ISO "given", as you initially described the situation?

If by "given" you mean "fixed in advance", then:

If ISO is "given", "under exposure" (or "over exposure") describe the
situation correctly.

If the exposure is "given", then "under exposure" (etc) are also
appropriate because of historical use (even today ISO is not as
variable as exposure), but also because the assumptions are known. But
if you don't like it, use "under amplified", "lack of gain" or
something and no one will quibble.
Anonymous
August 7, 2005 4:13:52 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

David J Taylor wrote:

> While we are talking about terms, I find JPS's use of the term
> "posterisation" confusing at best, and meaningless in a signal (or image)
> processing context. I think what he means may be "quantisation", the fact
> that an infinite range of analog values must be represented by a limited
> range of digital levels. Normally, sufficient digital levels are
> available and it is the accuracy of the analog signal which determines the
> signal-to-noise ratio of the system. However, if the quantisation steps
> are too large, it becomes the quantisation process itself which limits the
> signal-to-noise ratio.

Yes, when a photo-geek starts talking about "posterization", they are
referring to what anyone with a background in signal processing would
call "quantization noise".

http://en.wikipedia.org/wiki/Quantization_noise

Many fields have a maddening "terminological thicket" to penetrate...
Related resources
Anonymous
August 7, 2005 4:18:31 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

JPS@no.komm wrote:

> After spending some time under the hood, so to speak, of RAW capture and
> data, I find it increasingly difficult to use the term "exposure" to
> refer to the relative degree of photon saturation in a JPEG or RAW at a
> given ISO. The analog to slide film exposure is actually the analog
> exposure on the sensor; the ISO settings of the digital camera are like
> setting different ranges of exposure in a slide to be digitized by a
> scanner.
>
> Why then, do we call utilizing the specified range "exposure". I often
> substitute the word "digitized" in this context, but it draws strange
> reactions from some people.

This is photography so photographic terms apply.

There is nothing wrong with the word exposure for digital capture.
After all the sensor is exposed to light for a period of time and during
that time the sensors 'charge up' from the expsoure and then the data is
recorded.

From Webster's:
4 : a piece or section of sensitized material (as film) on which an
exposure is or can be made <36 exposures per roll>

While they state film, the "material" can be anything that is sensitive
to photons including the sensors (sites) that make up the sensor array
in the camera.

Regarding RAW processing, it is analogous (at a high enough level) to
the adjustments one might make in the darkroom (pushing, puling,
burning, dodging, pre-flashing the paper ... etc.)

For that matter, the same applies to scanners.

You may be right about the term "digitization" but you draw strange
looks becasue it is not the famillar term. And there really is nothing
wrong with the term exposure. That word says is all: Time X aperture.

Cheers,
Alan

--
-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm
-- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm
-- [SI] gallery & rulz: http://www.pbase.com/shootin
-- e-meil: Remove FreeLunch.
Anonymous
August 7, 2005 5:55:14 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

<JPS@no.komm> wrote:

> The analog to slide film exposure is actually the analog exposure on the
> sensor; the ISO settings of the digital camera are like setting different
> ranges of exposure in a slide to be digitized by a scanner.
>
> Why then, do we call utilizing the specified range "exposure".

Because when taking a picture, the sensor is exposed to light. When not
taking the picture, the sensor isn't exposed to light, particularly when
it comes to DSLRs.
Anonymous
August 7, 2005 6:03:48 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

In article <l6ubf1tpl10dg2rhc7uc939e0q31bj342a@4ax.com>, JPS@no.komm
wrote:

> After spending some time under the hood, so to speak, of RAW capture and
> data, I find it increasingly difficult to use the term "exposure" to
> refer to the relative degree of photon saturation in a JPEG or RAW at a
> given ISO. The analog to slide film exposure is actually the analog
> exposure on the sensor; the ISO settings of the digital camera are like
> setting different ranges of exposure in a slide to be digitized by a
> scanner.


> Why then, do we call utilizing the specified range "exposure". I often
> substitute the word "digitized" in this context, but it draws strange
> reactions from some people.

In truth its not about exposure, analog or digital....its selective
contrast determination and what can be recorded within the parameters of
the hardware. That is; its More or less about what you wish to drop or
pick up when you select to use analog or digital. But only within a
narrow reference as given by the maker of film or the maker of the
camera.

Perhaps its more an issue of word choice for people less able to grasp
the concept. But you are exposing the sensor to light, so you are making
an exposure.

To answer you quite directly: for lack of using a better description
and to be concise.

--
LF Website @ http://members.verizon.net/~gregoryblank

"To announce that there must be no criticism of the President,
or that we are to stand by the President, right or wrong,
is not only unpatriotic and servile, but is morally treasonable
to the American public."--Theodore Roosevelt, May 7, 1918
Anonymous
August 7, 2005 6:24:57 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

In message <greg-CB3D30.10141607082005@news.verizon.net>,
Gregory Blank <greg@greg_____photo.com> wrote:

>In article <l6ubf1tpl10dg2rhc7uc939e0q31bj342a@4ax.com>, JPS@no.komm
>wrote:

>> After spending some time under the hood, so to speak, of RAW capture and
>> data, I find it increasingly difficult to use the term "exposure" to
>> refer to the relative degree of photon saturation in a JPEG or RAW at a
>> given ISO. The analog to slide film exposure is actually the analog
>> exposure on the sensor; the ISO settings of the digital camera are like
>> setting different ranges of exposure in a slide to be digitized by a
>> scanner.

>> Why then, do we call utilizing the specified range "exposure". I often
>> substitute the word "digitized" in this context, but it draws strange
>> reactions from some people.

>In truth its not about exposure, analog or digital....its selective
>contrast determination and what can be recorded within the parameters of
>the hardware. That is; its More or less about what you wish to drop or
>pick up when you select to use analog or digital. But only within a
>narrow reference as given by the maker of film or the maker of the
>camera.

>Perhaps its more an issue of word choice for people less able to grasp
>the concept. But you are exposing the sensor to light, so you are making
>an exposure.

>To answer you quite directly: for lack of using a better description
>and to be concise.

If someone decides that "ISO 100 gives the best quality" and gets an
image that utilizes only 1/16th of the RAW values available, they would
have had a much better image if they had the camera set to ISO 1600 with
the same aperture and shutter speed. I have a hard time saying that
they "under-exposed" the image; it makes more sense to say that they
under-digitized it (quantized it) by using too low of an ISO.
--

<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
John P Sheehy <JPS@no.komm>
><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><
Anonymous
August 7, 2005 6:59:09 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

In article <jl5cf15719g5ujakrprlg31ntijmido5r4@4ax.com>, JPS@no.komm
wrote:

> >To answer you quite directly: for lack of using a better description
> >and to be concise.
>
> If someone decides that "ISO 100 gives the best quality" and gets an
> image that utilizes only 1/16th of the RAW values available, they would
> have had a much better image if they had the camera set to ISO 1600 with
> the same aperture and shutter speed. I have a hard time saying that
> they "under-exposed" the image; it makes more sense to say that they
> under-digitized it (quantized it) by using too low of an ISO.


Thats a rather extreme example,... & it seems unlikely.

That is: is it better for noise, range and color? Or does one make the
choice to keep two and drop one from the equation? Because if its better
for all three you would only need one ISO setting and not any
supplemental light sources.

You add flash/or lights as needed to make the Iso 100 image. Digital
does not solve the problems that exist beyond the scope of the camera-
lighting. & more likely It never will. Lighting is separate set of
issues and require knowledge. I can't seem to state this enough to
people, its something schools should teach :) 

--
LF Website @ http://members.verizon.net/~gregoryblank

"To announce that there must be no criticism of the President,
or that we are to stand by the President, right or wrong,
is not only unpatriotic and servile, but is morally treasonable
to the American public."--Theodore Roosevelt, May 7, 1918
Anonymous
August 7, 2005 7:56:35 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

In message <greg-CF27C5.11093807082005@news.verizon.net>,
Gregory Blank <greg@greg_____photo.com> wrote:

>In article <jl5cf15719g5ujakrprlg31ntijmido5r4@4ax.com>, JPS@no.komm
>wrote:
>
>> >To answer you quite directly: for lack of using a better description
>> >and to be concise.
>>
>> If someone decides that "ISO 100 gives the best quality" and gets an
>> image that utilizes only 1/16th of the RAW values available, they would
>> have had a much better image if they had the camera set to ISO 1600 with
>> the same aperture and shutter speed. I have a hard time saying that
>> they "under-exposed" the image; it makes more sense to say that they
>> under-digitized it (quantized it) by using too low of an ISO.

>Thats a rather extreme example,... & it seems unlikely.

If you think that's unlikely, you haven't been reading people's posts,
or DPReview. The problem of people under-digitizing at ISO 100 is
epidemic, because of the myth that ISO settings cause noise.

"Why is the sky so noisy in my ISO 100 picture" is a common question.
Of course, it is not just sensor-noisy, it's also highly posterized as
well, and would have looked better if taken at a higher ISO setting,
even with the same aperture and shutter speed. If they were using a
tripod, of course, they could have had a good digitization at a higher
absolute exposure at a lower ISO. I personally don't use ISO 100 very
often, but aim for ISO 200 if I can do it with a full digitization.
Blooming looms just above RAW value 4095 at ISO 100. In my experiments,
the trade-off between sterile posterization and noise indicates that
there is very little value in using ISO 100 over ISO 200 on my Canon
20D. The shadows are of approximately equal worth.

>That is: is it better for noise, range and color? Or does one make the
>choice to keep two and drop one from the equation? Because if its better
>for all three you would only need one ISO setting and not any
>supplemental light sources.

>You add flash/or lights as needed to make the Iso 100 image.

Not in available light photography, you don't.

>Digital
>does not solve the problems that exist beyond the scope of the camera-
>lighting. & more likely It never will. Lighting is separate set of
>issues and require knowledge.

Optimal lighting is different for digital and film. Color film
generally wants to see sunlight or tungsten, depending on the film.
Most digitals have neither sunlight nor tungsten as their native white
balance. The native balances generally run from magenta to pink
lighting with RGB bayer cameras. My Canon DSLRs get the best images
with lighting that is a stop stronger red than green, and a half stop
stronger blue than green.

>I can't seem to state this enough to
>people, its something schools should teach :) 

Should be taught specific to digital, but I doubt that there are many
teachjers who know the difference.

Available-light photography can only be improved by maximizing exposure
without clipping, or using filters over the lens if there is enough
light.

--

<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
John P Sheehy <JPS@no.komm>
><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><
Anonymous
August 7, 2005 8:38:33 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

In message <dd5c8m$mft$1@inews.gazeta.pl>,
Alan Browne <alan.browne@FreeLunchVideotron.ca> wrote:

>This is photography so photographic terms apply.

>There is nothing wrong with the word exposure for digital capture.
>After all the sensor is exposed to light for a period of time and during
>that time the sensors 'charge up' from the expsoure and then the data is
>recorded.
>
> From Webster's:
>4 : a piece or section of sensitized material (as film) on which an
>exposure is or can be made <36 exposures per roll>
>
>While they state film, the "material" can be anything that is sensitive
>to photons including the sensors (sites) that make up the sensor array
>in the camera.

I didn't say that the term exposure doesn't apply at all. I said that
it wasn't a good way to describe the relative digitization at an ISO
setting.

>Regarding RAW processing, it is analogous (at a high enough level) to
>the adjustments one might make in the darkroom (pushing, puling,
>burning, dodging, pre-flashing the paper ... etc.)
>
>For that matter, the same applies to scanners.
>
>You may be right about the term "digitization" but you draw strange
>looks becasue it is not the famillar term. And there really is nothing
>wrong with the term exposure. That word says is all: Time X aperture.

This is what I'm talking about; this is what I think exposure really
means (for a given subject intensity, of course). It is often used,
however, for the relative brightness of an image converted with 0
exposure adjustment, which, IMO, is more appropriately called
digitization.
--

<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
John P Sheehy <JPS@no.komm>
><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><
Anonymous
August 7, 2005 8:38:50 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

JPS@no.komm wrote:


> I didn't say that the term exposure doesn't apply at all. I said that
> it wasn't a good way to describe the relative digitization at an ISO
> setting.

And most photographers will say: "So what? Now let's meter the scene
and set the exposure."


>>You may be right about the term "digitization" but you draw strange
>>looks becasue it is not the famillar term. And there really is nothing
>>wrong with the term exposure. That word says is all: Time X aperture.
>
>
> This is what I'm talking about; this is what I think exposure really
> means (for a given subject intensity, of course). It is often used,
> however, for the relative brightness of an image converted with 0
> exposure adjustment, which, IMO, is more appropriately called
> digitization.

In engineering terms, sure. But few dedicated photographers are
engineers. And most engineers who photograph as a hobby or interest do
so as a pastime, not as an extension of their engineering life. Some
nerds excepted (not all engineers are nerds, far from it in my experience).

As to the 0 exposure adjustment, there is none, really. None of the
sensors have an ISO 134.6767 (whatever) setting where they are unity
gain. Few of the cameras record an ISO 100 (or other) sensitivity the
same as their competitors (or even models from the same co.)

You're raising a total non issue. Or at best an issue that is
meaningful to you and very few others. Photographers make photography
and think in photographic terms.

I know engineers who paint as well, but they don't talk about "pigment
carrying linum usitatissimum oils". They talk about oil paints.

Cheers,
Alan

--
-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm
-- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm
-- [SI] gallery & rulz: http://www.pbase.com/shootin
-- e-meil: Remove FreeLunch.
Anonymous
August 7, 2005 8:39:32 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

Alan Browne wrote:
> JPS@no.komm wrote:
>
>> After spending some time under the hood, so to speak, of RAW capture
>> and data, I find it increasingly difficult to use the term
>> "exposure" to refer to the relative degree of photon saturation in a
>> JPEG or RAW at a given ISO. The analog to slide film exposure is
>> actually the analog exposure on the sensor; the ISO settings of the
>> digital camera are like setting different ranges of exposure in a
>> slide to be digitized by a scanner.
>>
>> Why then, do we call utilizing the specified range "exposure". I
>> often substitute the word "digitized" in this context, but it draws
>> strange reactions from some people.
>
> This is photography so photographic terms apply.
>
> There is nothing wrong with the word exposure for digital capture.
> After all the sensor is exposed to light for a period of time and
> during that time the sensors 'charge up' from the expsoure and then
> the data is recorded.
>
> From Webster's:
> 4 : a piece or section of sensitized material (as film) on which an
> exposure is or can be made <36 exposures per roll>
>
> While they state film, the "material" can be anything that is
> sensitive to photons including the sensors (sites) that make up the
> sensor array in the camera.
>
> Regarding RAW processing, it is analogous (at a high enough level) to
> the adjustments one might make in the darkroom (pushing, puling,
> burning, dodging, pre-flashing the paper ... etc.)
>
> For that matter, the same applies to scanners.
>
> You may be right about the term "digitization" but you draw strange
> looks becasue it is not the famillar term. And there really is
> nothing wrong with the term exposure. That word says is all: Time X
> aperture.
> Cheers,
> Alan

I would agree with Alan that "exposure" is a reasonable term to use, where
the product of the light intensity and a time determines the number of
photo-electrons which will be created.

To me, as a signal-processing person, digitisation is a process that
happens at a single point in time, where an analog value is converted into
a digital one. It has nothing to do with the actual value obtained, nor
the time taken to acquire that value.

While we are talking about terms, I find JPS's use of the term
"posterisation" confusing at best, and meaningless in a signal (or image)
processing context. I think what he means may be "quantisation", the fact
that an infinite range of analog values must be represented by a limited
range of digital levels. Normally, sufficient digital levels are
available and it is the accuracy of the analog signal which determines the
signal-to-noise ratio of the system. However, if the quantisation steps
are too large, it becomes the quantisation process itself which limits the
signal-to-noise ratio.

An example from audio might be that when sounds are digitised to 8-bits
rather than 16-bits, there is an added roughness to the sound. An error
is introduced which depends on signal level. In an image, too few bits
would show as a contouring effect where what should be a smooth transition
of brightness levels instead appears as a finite series of perceptibly
different brightness bands.

I would call this effect quantisation errors, or more specifically errors
due to using too few bits to represent the signal. Is this what you mean
by posterisation?

David
Anonymous
August 7, 2005 8:46:21 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

David J Taylor wrote:

> An example from audio might be that when sounds are digitised to 8-bits
> rather than 16-bits, there is an added roughness to the sound. An error
> is introduced which depends on signal level. In an image, too few bits
> would show as a contouring effect where what should be a smooth transition
> of brightness levels instead appears as a finite series of perceptibly
> different brightness bands.
>
> I would call this effect quantisation errors, or more specifically errors
> due to using too few bits to represent the signal. Is this what you mean
> by posterisation?

I brought up the issue of quantization noise many months (over a year?)
ago on rpd. It seemed to sail over the heads of just about everyone. I
too did some signal processing work 10 or so years ago synthesizing
complex radar wave forms in realtime. Quantization noise was not a
problem in the synthesis (16 bit DAC) but for the system under test with
a very high dyncamic range, it was a serious issue when the SNR was very
low.

For photography, the quantization noise is the noise we typically see at
high ISO settings in the shaddow areas of the image. Some liken this
(erroneously) to film grain. However film grain has dimension across
the image (x,y), as well as in color error (z), whereas quantization
noise is dynamic (z) (color) only in digital cameras.

Cheers,
Alan.


--
-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm
-- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm
-- [SI] gallery & rulz: http://www.pbase.com/shootin
-- e-meil: Remove FreeLunch.
Anonymous
August 7, 2005 9:24:11 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

JPS@no.komm wrote:
[]
> This is what I'm talking about; this is what I think exposure really
> means (for a given subject intensity, of course). It is often used,
> however, for the relative brightness of an image converted with 0
> exposure adjustment, which, IMO, is more appropriately called
> digitization.

In signal processing we might use the term "headroom". If the sound level
a system could capture was 8, and the loudest sound to be recorded was 4,
then we might say the headroom was 6dB (an engineering term for a factor
of 2 in linear voltage or current terms). Similarly, if the maximum value
from your image sensor is 4095, and the white level of a particular image
were 2047, then you also have a factor of two headroom. Were any part of
the image to exceed the maximum value - specular highlight for example -
we would say the value was clipped.

"Digitisation" is simply the process of converting analog to digital.

David
Anonymous
August 7, 2005 10:13:22 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

In article <db9cf1t1pl59no9ap9b4mfgp2chc80rc2d@4ax.com>, JPS@no.komm
wrote:

> >Thats a rather extreme example,... & it seems unlikely.
>
> If you think that's unlikely, you haven't been reading people's posts,
> or DPReview. The problem of people under-digitizing at ISO 100 is
> epidemic, because of the myth that ISO settings cause noise.

Up until this moment:

The only posts I have read are yours and mine regarding this
thread. Hey under exposure is under exposure. On film you get nothing.


> "Why is the sky so noisy in my ISO 100 picture" is a common question.
> Of course, it is not just sensor-noisy, it's also highly posterized as
> well, and would have looked better if taken at a higher ISO setting,
> even with the same aperture and shutter speed. If they were using a
> tripod, of course, they could have had a good digitization at a higher
> absolute exposure at a lower ISO. I personally don't use ISO 100 very
> often, but aim for ISO 200 if I can do it with a full digitization.
> Blooming looms just above RAW value 4095 at ISO 100. In my experiments,
> the trade-off between sterile posterization and noise indicates that
> there is very little value in using ISO 100 over ISO 200 on my Canon
> 20D. The shadows are of approximately equal worth.
>
> >That is: is it better for noise, range and color? Or does one make the
> >choice to keep two and drop one from the equation? Because if its better
> >for all three you would only need one ISO setting and not any
> >supplemental light sources.
>
> >You add flash/or lights as needed to make the Iso 100 image.
>
> Not in available light photography, you don't.

So knowledge of basic photography is a good thing if one wants to make
good pictures.

> >Digital
> >does not solve the problems that exist beyond the scope of the camera-
> >lighting. & more likely It never will. Lighting is separate set of
> >issues and require knowledge.
>
> Optimal lighting is different for digital and film.

To a degree maybe, but fairly close for slide film and the sensor.

> Color film
> generally wants to see sunlight or tungsten, depending on the film.

> Most digitals have neither sunlight nor tungsten as their native white
> balance. The native balances generally run from magenta to pink
> lighting with RGB bayer cameras. My Canon DSLRs get the best images
> with lighting that is a stop stronger red than green, and a half stop
> stronger blue than green.

Like I stated prior it relative to what the maker imparts.

> >I can't seem to state this enough to
> >people, its something schools should teach :) 
>
> Should be taught specific to digital, but I doubt that there are many
> teachjers who know the difference.

Should be taught specific to what works it incorporates many
discipline fields. Video, film and digital the principles of light
adjustment should be considered a portion of many curriculum including
architectural design,...they maybe in that but I am just stating that
its an important branch of making pictures.

> Available-light photography can only be improved by maximizing exposure
> without clipping, or using filters over the lens if there is enough
> light.

It all boils down to understanding, that is knowing when to use that
filter and when not,...its the same for film images.

--
LF Website @ http://members.verizon.net/~gregoryblank

"To announce that there must be no criticism of the President,
or that we are to stand by the President, right or wrong,
is not only unpatriotic and servile, but is morally treasonable
to the American public."--Theodore Roosevelt, May 7, 1918
Anonymous
August 8, 2005 12:04:09 AM

Archived from groups: rec.photo.digital.slr-systems (More info?)

<JPS@no.komm> wrote:

> If someone decides that "ISO 100 gives the best quality" and gets an
> image that utilizes only 1/16th of the RAW values available, they would
> have had a much better image if they had the camera set to ISO 1600 with
> the same aperture and shutter speed. I have a hard time saying that
> they "under-exposed" the image; it makes more sense to say that they
> under-digitized it (quantized it) by using too low of an ISO.

The thing is, you're the only person I have *ever* seen talk in terms of
"absolute exposure", or compare different ISOs at the same aperture and
shutter speed. I'm not saying that's not a valid way to think about it,
I just don't see how it could be useful to me. ISO 100 *does* give the
best quality, as long as you don't underexpose it, and it's a given that
a proper exposure at an elevated ISO rating is better than underexposing
at ISO 100.

Now, there is another angle to the whole thing, and that is the 12-bit
A/D conversion. We've discussed previously how the dynamic range of
current sensors is not limited by the sensor's capability, but rather
by the A/D conversion. This presents an interesting situation. Imagine
that a camera used 16-bit A/D conversion. Imagine that the extra range
actually *did* use all of the data available from the sensor. You now
have a situation where higher ISO settings are meaningless, and the
camera would have to be marketed as (for example) ISO 100 with *no*
higher settings. Imagine the outcry!

The simple fact that higher ISO settings exist and are useful tells
us that the A/D conversion is not using all of the data the sensor is
providing. Higher ISOs are accomplished by amplifying the signal.
If you can usefully amplify the signal to ISO 800, that means there
was a signal there in the first place to amplify, one that *could*
have been used at ISO 100, but was ignored at that setting. If no
data from the sensor were ignored, there would be nothing left to
amplify, and ISO 200 would just be ISO 100 with one stop less of
range and no actual advantage whatsoever. That is, it *would* be
better to underexpose at ISO 100 and then push it in processing, to
avoid the amplification step.

So, it seems that either 16-bit A/D conversion is more complicated
to put into a camera than it sounds, or we are having our dynamic
range artificially limited in order to allow camera manufacturers
to say that their cameras can go to ISO 800 or whatever.

Of course, if the sensor can provide more range than a 16-bit
conversion would need, then there would still be room for higher
ISO settings.

--
Jeremy | jeremy@exit109.com
Anonymous
August 8, 2005 12:39:13 AM

Archived from groups: rec.photo.digital.slr-systems (More info?)

On Sun, 07 Aug 2005 12:13:51 GMT, JPS@no.komm wrote:

>After spending some time under the hood, so to speak, of RAW capture and
>data, I find it increasingly difficult to use the term "exposure" to
>refer to the relative degree of photon saturation in a JPEG or RAW at a
>given ISO. The analog to slide film exposure is actually the analog
>exposure on the sensor; the ISO settings of the digital camera are like
>setting different ranges of exposure in a slide to be digitized by a
>scanner.
>
>Why then, do we call utilizing the specified range "exposure". I often
>substitute the word "digitized" in this context, but it draws strange
>reactions from some people.

I prefer the term "imaging".


*********************************************************

"It looked like the sort of book described in library
catalogues as "slightly foxed", although it would be
more honest to admit that it looked as though it had
been badgered, wolved and possibly beared as well."

_Light Fantastic_
Terry Pratchett
Anonymous
August 8, 2005 4:42:10 AM

Archived from groups: rec.photo.digital.slr-systems (More info?)

Alan Browne wrote:
[]
> I brought up the issue of quantization noise many months (over a
> year?) ago on rpd. It seemed to sail over the heads of just about
> everyone. I too did some signal processing work 10 or so years ago
> synthesizing complex radar wave forms in realtime. Quantization
> noise was not a problem in the synthesis (16 bit DAC) but for the
> system under test with a very high dyncamic range, it was a serious
> issue when the SNR was very low.
>
> For photography, the quantization noise is the noise we typically see
> at high ISO settings in the shaddow areas of the image. Some liken
> this (erroneously) to film grain. However film grain has dimension
> across the image (x,y), as well as in color error (z), whereas
> quantization noise is dynamic (z) (color) only in digital cameras.
>
> Cheers,
> Alan.

Thanks, Alan.

I would be surprised if quantization noise were an issue at low SNR (i.e.
high ISO settings) in a digital camera, though, as the signal is amplified
before the ADC, so that the photon noise should swamp the quantisation
noise. However, I haven't sat down and done the sums....

Cheers,
David
Anonymous
August 8, 2005 4:42:11 AM

Archived from groups: rec.photo.digital.slr-systems (More info?)

David J Taylor wrote:

> Alan Browne wrote:
> []
>
>>I brought up the issue of quantization noise many months (over a
>>year?) ago on rpd. It seemed to sail over the heads of just about
>>everyone. I too did some signal processing work 10 or so years ago
>>synthesizing complex radar wave forms in realtime. Quantization
>>noise was not a problem in the synthesis (16 bit DAC) but for the
>>system under test with a very high dyncamic range, it was a serious
>>issue when the SNR was very low.
>>
>>For photography, the quantization noise is the noise we typically see
>>at high ISO settings in the shaddow areas of the image. Some liken
>>this (erroneously) to film grain. However film grain has dimension
>>across the image (x,y), as well as in color error (z), whereas
>>quantization noise is dynamic (z) (color) only in digital cameras.
>>
>>Cheers,
>>Alan.
>
>
> Thanks, Alan.
>
> I would be surprised if quantization noise were an issue at low SNR (i.e.
> high ISO settings) in a digital camera, though, as the signal is amplified
> before the ADC, so that the photon noise should swamp the quantisation
> noise. However, I haven't sat down and done the sums....

A good point, however a closeup examination of photographs at 1600 and
3200 show discrete pixel jumps that are similar in nature to the effects
of quantization noise. It is (as you say) disguised by other things in
the chain (ADC, unknown local pre-ADC amps), but it does indeed bear
resemblance to quantization noise.

Cheers,
Alan.
--
-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm
-- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm
-- [SI] gallery & rulz: http://www.pbase.com/shootin
-- e-meil: Remove FreeLunch.
Anonymous
August 8, 2005 4:53:10 AM

Archived from groups: rec.photo.digital.slr-systems (More info?)

In message <%srJe.84154$G8.27258@text.news.blueyonder.co.uk>,
"David J Taylor"
<david-taylor@blueyonder.co.not-this-bit.nor-this-part.uk.invalid>
wrote:

>
>JPS@no.komm wrote:
>[]
>> This is what I'm talking about; this is what I think exposure really
>> means (for a given subject intensity, of course). It is often used,
>> however, for the relative brightness of an image converted with 0
>> exposure adjustment, which, IMO, is more appropriately called
>> digitization.
>
>In signal processing we might use the term "headroom". If the sound level
>a system could capture was 8, and the loudest sound to be recorded was 4,
>then we might say the headroom was 6dB (an engineering term for a factor
>of 2 in linear voltage or current terms). Similarly, if the maximum value
>from your image sensor is 4095, and the white level of a particular image
>were 2047, then you also have a factor of two headroom. Were any part of
>the image to exceed the maximum value - specular highlight for example -
>we would say the value was clipped.
>
>"Digitisation" is simply the process of converting analog to digital.

Implicit in everything I say here about digitization is the quality
factor; "well-digitized", "poorly digitized", etc.

A RAW file with the highest value as 600 in a specular highlight is
poorly digitized. A maximum of 2048 is not as well-digitized as a
maximum of 3800.
--

<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
John P Sheehy <JPS@no.komm>
><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><
Anonymous
August 8, 2005 5:07:29 AM

Archived from groups: rec.photo.digital.slr-systems (More info?)

In message <8PqJe.84115$G8.66978@text.news.blueyonder.co.uk>,
"David J Taylor"
<david-taylor@blueyonder.co.not-this-bit.nor-this-part.uk.invalid>
wrote:

>While we are talking about terms, I find JPS's use of the term
>"posterisation" confusing at best, and meaningless in a signal (or image)
>processing context.

I've understood "quantization" to be a specific kind of "posterization";
one where the values are integers or multiples of integers, whereas
posterization can include things like 1 2 4 5 7 8 10 11, etc.

"Quantization" would be clearer, I suppose, even if this is correct. I
see the word posterized much more than I do quantized, nowadays, and
assumed the former was more in common usage. People almost always refer
to an image with too few color levels to represent smooth gradients as
"posterized".
--

<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
John P Sheehy <JPS@no.komm>
><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><
Anonymous
August 8, 2005 5:23:21 AM

Archived from groups: rec.photo.digital.slr-systems (More info?)

JPS@no.komm wrote:
> In message <8PqJe.84115$G8.66978@text.news.blueyonder.co.uk>,
> "David J Taylor"
> <david-taylor@blueyonder.co.not-this-bit.nor-this-part.uk.invalid>
> wrote:
>
>> While we are talking about terms, I find JPS's use of the term
>> "posterisation" confusing at best, and meaningless in a signal (or
>> image) processing context.
>
> I've understood "quantization" to be a specific kind of
> "posterization"; one where the values are integers or multiples of
> integers, whereas posterization can include things like 1 2 4 5 7 8
> 10 11, etc.

That's probably fair comment - most quantisers indeed use linear steps.

> "Quantization" would be clearer, I suppose, even if this is correct.
> I see the word posterized much more than I do quantized, nowadays, and
> assumed the former was more in common usage. People almost always
> refer to an image with too few color levels to represent smooth
> gradients as "posterized".

Yes, sometimes I see this effect used in television for, er, "artistic"
purposes. I've never understood why, personally! Were, instead, I
referring to something where this was a problem, I would simply say there
were too few quantisation levels, or that the quantisation step was too
large. Different steps in each of the RGB channels can even introduce
colour casts.

David
Anonymous
August 8, 2005 5:29:32 AM

Archived from groups: rec.photo.digital.slr-systems (More info?)

In message <pascf1dld2tfu1rht7l6jolba2k94ger9n@4ax.com>,
John A. Stovall <johnastovall@earthlink.net> wrote:

>On Sun, 07 Aug 2005 12:13:51 GMT, JPS@no.komm wrote:
>
>>After spending some time under the hood, so to speak, of RAW capture and
>>data, I find it increasingly difficult to use the term "exposure" to
>>refer to the relative degree of photon saturation in a JPEG or RAW at a
>>given ISO. The analog to slide film exposure is actually the analog
>>exposure on the sensor; the ISO settings of the digital camera are like
>>setting different ranges of exposure in a slide to be digitized by a
>>scanner.
>>
>>Why then, do we call utilizing the specified range "exposure". I often
>>substitute the word "digitized" in this context, but it draws strange
>>reactions from some people.
>
>I prefer the term "imaging".

Well, it's part of "imaging", but "imaging" is just too broad, IMO.
--

<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
John P Sheehy <JPS@no.komm>
><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><
Anonymous
August 8, 2005 5:35:28 AM

Archived from groups: rec.photo.digital.slr-systems (More info?)

In message <1h0xft4.1bb4hr4h6wwlcN%usenet@mile23.c0m>,
usenet@mile23.c0m (Paul Mitchum) wrote:

><JPS@no.komm> wrote:
>
>> The analog to slide film exposure is actually the analog exposure on the
>> sensor; the ISO settings of the digital camera are like setting different
>> ranges of exposure in a slide to be digitized by a scanner.
>>
>> Why then, do we call utilizing the specified range "exposure".
>
>Because when taking a picture, the sensor is exposed to light. When not
>taking the picture, the sensor isn't exposed to light, particularly when
>it comes to DSLRs.

Thanks for that worthless response. Did you even bother to try to
understand what my question was?

At any given ISO setting, only part of the range of sensor exposure is
converted to meaningful numbers. The sensor, I understand, receives an
exposure. How then do we call how well the smaller, digitized range is
utilized, "exposure". That was my pondering.
--

<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
John P Sheehy <JPS@no.komm>
><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><
Anonymous
August 8, 2005 5:35:29 AM

Archived from groups: rec.photo.digital.slr-systems (More info?)

<JPS@no.komm> wrote:

> In message <1h0xft4.1bb4hr4h6wwlcN%usenet@mile23.c0m>,
> usenet@mile23.c0m (Paul Mitchum) wrote:
>
> ><JPS@no.komm> wrote:
> >
> >> The analog to slide film exposure is actually the analog exposure on the
> >> sensor; the ISO settings of the digital camera are like setting different
> >> ranges of exposure in a slide to be digitized by a scanner.
> >>
> >> Why then, do we call utilizing the specified range "exposure".
> >
> >Because when taking a picture, the sensor is exposed to light. When not
> >taking the picture, the sensor isn't exposed to light, particularly when
> >it comes to DSLRs.
>
> Thanks for that worthless response.

Welcome to usenet. :-)

> Did you even bother to try to understand what my question was?

You were quibbling over the word 'exposure.' The rest of the question
was about the dynamic range of the data resulting from the process of
exposure, not whether it *was* an exposure.

> At any given ISO setting, only part of the range of sensor exposure is
> converted to meaningful numbers. The sensor, I understand, receives an
> exposure. How then do we call how well the smaller, digitized range is
> utilized, "exposure". That was my pondering.

What you're asking is why we call a subset of the dynamic range falling
on the sensor an 'exposure.' And I answered it: Because it's exposed to
light.

You'd prefer there be an exposure, which is the sensor being shown the
light, and then another name for the process of amplification (per ISO),
conversion to digital, and optionally in-camera processing. Or perhaps a
name for each of those things. Well, there you go: Amplification, A/D
conversion, and processing.

What's the problem?
Anonymous
August 8, 2005 5:35:30 AM

Archived from groups: rec.photo.digital.slr-systems (More info?)

Paul Mitchum wrote:


>
>>Did you even bother to try to understand what my question was?
>
>
> You were quibbling over the word 'exposure.' The rest of the question
> was about the dynamic range of the data resulting from the process of
> exposure, not whether it *was* an exposure.

Paul, I admire your restraint in answering the questions above.

>
> What's the problem?

None at all.

Cheers,
Alan


--
-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm
-- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm
-- [SI] gallery & rulz: http://www.pbase.com/shootin
-- e-meil: Remove FreeLunch.
Anonymous
August 8, 2005 10:25:50 AM

Archived from groups: rec.photo.digital.slr-systems (More info?)

JPS@no.komm wrote:
[]
> At any given ISO setting, only part of the range of sensor exposure is
> converted to meaningful numbers. The sensor, I understand, receives
> an exposure. How then do we call how well the smaller, digitized
> range is utilized, "exposure". That was my pondering.

You could use "dynamic range used" or "headroom".

- the image occupies 75% of the available dynamic range if the maximum
value is 3072 but the ADC maximum is 4095

- there is a headroom of 20% if the maximum signal is 800 and the ADC
limit is 1000

As dynamic range can also be used to describe the ratio of maximum to
minimum signal, I prefer the term "headroom".

I do now, however, see what you mean by exposure - in a histogram, that
part of the curve which is non-zero. It's the occupied part of the total
system dynamic range.

David
Anonymous
August 8, 2005 10:44:03 AM

Archived from groups: rec.photo.digital.slr-systems (More info?)

Alan Browne wrote:
[]
> A good point, however a closeup examination of photographs at 1600 and
> 3200 show discrete pixel jumps that are similar in nature to the
> effects of quantization noise. It is (as you say) disguised by other
> things in the chain (ADC, unknown local pre-ADC amps), but it does
> indeed bear resemblance to quantization noise.
>
> Cheers,
> Alan.

To get the higher ISOs, for example, and using simple numbers:

- in normal mode, the converter digitises a voltage range of 0..4V to a
digital range of 0..4000. This means it must be accurate to 0.001V. The
ADC will work by comparing a signal with a 4V reference value.

- in ISO 3200 mode, the signal range is now just 0..1V. To do this, you
could

either:

-- quadruple the values from the ADC, turning it into a device digitising
0..1V to a digital range of 0..1000, but multiplying each result by 4, so
that 0..1V returns digital values of 0..4000. The quantisation steps are
still 0.001V, but as the digital values are quadrupled, the digital levels
in the image will be in steps of 4; 0, 4, 8, 12, etc.

or:

-- reduce the reference voltage in the converter so that it measures the
analog voltage against a 1V reference, but still returns values 0..4000.
The quantisation steps are now 0.0025V. Whilst the analog accuracy of the
converter may not justify the full 0,0025V steps, digitising this way may
produce a slightly more accurate result than simply quadrupling the
values. The digital levels will still be in steps of 1.

Perhaps there is a simple "double the output of the converter" happening
in cameras where this quantisation is observed? Perhaps both methods are
used to get two extra ISO steps?

Cheers,
David
Anonymous
August 8, 2005 11:52:34 AM

Archived from groups: rec.photo.digital.slr-systems (More info?)

In message <dd6fts$pbf$1@inews.gazeta.pl>,
Alan Browne <alan.browne@FreeLunchVideotron.ca> wrote:

>A good point, however a closeup examination of photographs at 1600 and
>3200 show discrete pixel jumps that are similar in nature to the effects
>of quantization noise. It is (as you say) disguised by other things in
>the chain (ADC, unknown local pre-ADC amps), but it does indeed bear
>resemblance to quantization noise.

If you want to see noise that is mainly quantization from the
digitization, just crank up the shadows of an under-exposed ISO 100
image. The high-ISO shadows are a combination of that and sensor noise
with a tad of amplifier noise.
--

<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
John P Sheehy <JPS@no.komm>
><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><
Anonymous
August 8, 2005 12:54:22 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

JPS@no.komm wrote:
> In message <dd6fts$pbf$1@inews.gazeta.pl>,
> Alan Browne <alan.browne@FreeLunchVideotron.ca> wrote:
>
>> A good point, however a closeup examination of photographs at 1600
>> and 3200 show discrete pixel jumps that are similar in nature to the
>> effects of quantization noise. It is (as you say) disguised by
>> other things in the chain (ADC, unknown local pre-ADC amps), but it
>> does indeed bear resemblance to quantization noise.
>
> If you want to see noise that is mainly quantization from the
> digitization, just crank up the shadows of an under-exposed ISO 100
> image. The high-ISO shadows are a combination of that and sensor
> noise with a tad of amplifier noise.

Yes, of course, the darkest values at ISO 100 would have the least photon
noise and therefore be the best places to see quantisation noise.

It still surprises me a little: back of the envelope sum: well capacity
32K, quantised to 4K values, so each quantisation step is 8 levels. Scene
dynamic range 1000:1 so smallest value about 32. Photo-limited noise sqrt
(32) is 5-6 levels. OK, so the quantisation step under those extreme
conditions is of the same order as the photon-limited noise. No longer
surprised!

David
Anonymous
August 8, 2005 8:51:27 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

David J Taylor wrote:
> Perhaps there is a simple "double the output of the converter"
> happening in cameras where this quantisation is observed? Perhaps
> both methods are used to get two extra ISO steps?

Yuk, I hope not.

It would be possible to test this but I'd rather take photos :-)

-Mike
Anonymous
August 8, 2005 8:51:28 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

Mike Warren wrote:
> David J Taylor wrote:
>> Perhaps there is a simple "double the output of the converter"
>> happening in cameras where this quantisation is observed? Perhaps
>> both methods are used to get two extra ISO steps?
>
> Yuk, I hope not.
>
> It would be possible to test this but I'd rather take photos :-)
>
> -Mike

It may be that the realistic maximum speed is ISO 800, but marketing
demands ISO 1600 and ISO 3200. Nothing extra to be extracted from the
sensor, so just double the ADC output and perhaps dither it a little to
disguise the action?

David
Anonymous
August 8, 2005 8:51:29 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

In message <UnDJe.84457$G8.53162@text.news.blueyonder.co.uk>,
"David J Taylor"
<david-taylor@blueyonder.co.not-this-bit.nor-this-part.uk.invalid>
wrote:

>Mike Warren wrote:
>> David J Taylor wrote:
>>> Perhaps there is a simple "double the output of the converter"
>>> happening in cameras where this quantisation is observed? Perhaps
>>> both methods are used to get two extra ISO steps?
>>
>> Yuk, I hope not.
>>
>> It would be possible to test this but I'd rather take photos :-)
>>
>> -Mike
>
>It may be that the realistic maximum speed is ISO 800, but marketing
>demands ISO 1600 and ISO 3200. Nothing extra to be extracted from the
>sensor, so just double the ADC output and perhaps dither it a little to
>disguise the action?

That's what the Canon 10D does, although it seems that its 3200 is 1600
pushed, and its 1600 is 800 pushed, so 3200 is not totally redundant, as
it has the only "1600-level" amplification. The dithering is enough to
fool a histogram, but if you look at the pattern of the least
significant bit, it is obvious:

http://www.pbase.com/jps_photo/image/38841732/original

--

<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
John P Sheehy <JPS@no.komm>
><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><
Anonymous
August 8, 2005 8:51:30 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

JPS@no.komm wrote:
> In message <UnDJe.84457$G8.53162@text.news.blueyonder.co.uk>,
> "David J Taylor"
> <david-taylor@blueyonder.co.not-this-bit.nor-this-part.uk.invalid>
> wrote:
>
>> Mike Warren wrote:
>>> David J Taylor wrote:
>>>> Perhaps there is a simple "double the output of the converter"
>>>> happening in cameras where this quantisation is observed? Perhaps
>>>> both methods are used to get two extra ISO steps?
>>>
>>> Yuk, I hope not.
>>>
>>> It would be possible to test this but I'd rather take photos :-)
>>>
>>> -Mike
>>
>> It may be that the realistic maximum speed is ISO 800, but marketing
>> demands ISO 1600 and ISO 3200. Nothing extra to be extracted from
>> the sensor, so just double the ADC output and perhaps dither it a
>> little to disguise the action?
>
> That's what the Canon 10D does, although it seems that its 3200 is
> 1600 pushed, and its 1600 is 800 pushed, so 3200 is not totally
> redundant, as it has the only "1600-level" amplification. The
> dithering is enough to fool a histogram, but if you look at the
> pattern of the least significant bit, it is obvious:
>
> http://www.pbase.com/jps_photo/image/38841732/original

If it really were dithering, I'm sure they could do much better than that!
They should use a dither pattern which isn't correlated with spatial
position or image brightness level.

Thanks, John.

David
Anonymous
August 9, 2005 8:53:58 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

<JPS@no.komm> wrote in message
news:D b9cf1t1pl59no9ap9b4mfgp2chc80rc2d@4ax.com...
> In message <greg-CF27C5.11093807082005@news.verizon.net>,
> Gregory Blank <greg@greg_____photo.com> wrote:
>
> >In article <jl5cf15719g5ujakrprlg31ntijmido5r4@4ax.com>, JPS@no.komm
> >wrote:
> >
> >> >To answer you quite directly: for lack of using a better description
> >> >and to be concise.
> >>
> >> If someone decides that "ISO 100 gives the best quality" and gets an
> >> image that utilizes only 1/16th of the RAW values available, they would
> >> have had a much better image if they had the camera set to ISO 1600
with
> >> the same aperture and shutter speed. I have a hard time saying that
> >> they "under-exposed" the image; it makes more sense to say that they
> >> under-digitized it (quantized it) by using too low of an ISO.
>

Is it just me, or are you getting the ISO's wrong here? ISO 100 allows for
the greatest amount of exposure and equals the best possible image quality
(except for on film where you can go much lower). People aren't being
mislead, it's exactly the same as in the film world. Faster films/iso's
means better low light/hand held photographs, but at the expense of
grain/noise.

--
Regards,
Matt Clara
www.mattclara.com
Anonymous
August 9, 2005 8:53:59 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

"Matt Clara" <no.emailz@this.guys.expense> wrote in message
news:Fc5Ke.15275$eC6.3486@fe05.news.easynews.com...
> <JPS@no.komm> wrote in message
> news:D b9cf1t1pl59no9ap9b4mfgp2chc80rc2d@4ax.com...
> > In message <greg-CF27C5.11093807082005@news.verizon.net>,
> > Gregory Blank <greg@greg_____photo.com> wrote:
> >
> > >In article <jl5cf15719g5ujakrprlg31ntijmido5r4@4ax.com>, JPS@no.komm
> > >wrote:
> > >
> > >> >To answer you quite directly: for lack of using a better description
> > >> >and to be concise.
> > >>
> > >> If someone decides that "ISO 100 gives the best quality" and gets an
> > >> image that utilizes only 1/16th of the RAW values available, they
would
> > >> have had a much better image if they had the camera set to ISO 1600
> with
> > >> the same aperture and shutter speed. I have a hard time saying that
> > >> they "under-exposed" the image; it makes more sense to say that they
> > >> under-digitized it (quantized it) by using too low of an ISO.
> >
>
> Is it just me, or are you getting the ISO's wrong here? ISO 100 allows
for
> the greatest amount of exposure and equals the best possible image quality
> (except for on film where you can go much lower). People aren't being
> mislead, it's exactly the same as in the film world. Faster films/iso's
> means better low light/hand held photographs, but at the expense of
> grain/noise.
>

JPS has discussed this many times in r.p.d.s-s. If you Google a little in
r.p.d.s-s. for noise, ISO, and his name as author, you'll see many of his
posts discussing ISO 100 and noise.

Greg
Anonymous
August 10, 2005 12:17:11 AM

Archived from groups: rec.photo.digital.slr-systems (More info?)

Matt Clara <no.emailz@this.guys.expense> wrote:
><JPS@no.komm> wrote:
>>
>> If someone decides that "ISO 100 gives the best quality" and gets an
>> image that utilizes only 1/16th of the RAW values available, they would
>> have had a much better image if they had the camera set to ISO 1600 with
>> the same aperture and shutter speed. I have a hard time saying that
>> they "under-exposed" the image; it makes more sense to say that they
>> under-digitized it (quantized it) by using too low of an ISO.
>
> Is it just me, or are you getting the ISO's wrong here? ISO 100 allows for
> the greatest amount of exposure and equals the best possible image quality
> (except for on film where you can go much lower).

He's referring to situations where the photographer chose to
underexpose rather than use a higher ISO. As long as the next higher
ISO step is made via analog gain, this is almost always a mistake.


> People aren't being mislead, it's exactly the same as in the film
> world. Faster films/iso's means better low light/hand held
> photographs, but at the expense of grain/noise.

This isn't strictly true; when you say that they mean "better low
light/hand held photographs", you're missing that this includes that
*they have less visible noise*. ISO800, correctly exposed, is less
noisy than ISO100 three stops under.

What makes this different from film is that you don't have to change
your sensor to get this effect, the way you would have to change your
film. You simply change a digitization setting. Thus, the "exposure"
of the sensor to light isn't actually changing at all, the silicon
isn't doing anything different at each ISO level.

One way of looking at it is to say that the sensor itself is always at
ISO1600 (or whatever the maximum analog setting is), and the camera
merely allows you to drop the gain to recover highlights and reduce
noise slightly. From that perspective, if a shot wasn't
"underexposed" if it were taken at ISO1600, it wasn't underexposed at
all... merely underdigitized by using too low an ISO setting. Does
that make more sense to you?

--
Zed Pobre <zed@resonant.org> a.k.a. Zed Pobre <zed@debian.org>
PGP key and fingerprint available on finger; encrypted mail welcomed.
Anonymous
August 10, 2005 1:34:49 AM

Archived from groups: rec.photo.digital.slr-systems (More info?)

In message <Fc5Ke.15275$eC6.3486@fe05.news.easynews.com>,
"Matt Clara" <no.emailz@this.guys.expense> wrote:

><JPS@no.komm> wrote in message
>news:D b9cf1t1pl59no9ap9b4mfgp2chc80rc2d@4ax.com...
>> In message <greg-CF27C5.11093807082005@news.verizon.net>,
>> Gregory Blank <greg@greg_____photo.com> wrote:
>>
>> >In article <jl5cf15719g5ujakrprlg31ntijmido5r4@4ax.com>, JPS@no.komm
>> >wrote:
>> >
>> >> >To answer you quite directly: for lack of using a better description
>> >> >and to be concise.
>> >>
>> >> If someone decides that "ISO 100 gives the best quality" and gets an
>> >> image that utilizes only 1/16th of the RAW values available, they would
>> >> have had a much better image if they had the camera set to ISO 1600
>> >> with
>> >> the same aperture and shutter speed. I have a hard time saying that
>> >> they "under-exposed" the image; it makes more sense to say that they
>> >> under-digitized it (quantized it) by using too low of an ISO.

>Is it just me, or are you getting the ISO's wrong here? ISO 100 allows for
>the greatest amount of exposure and equals the best possible image quality
>(except for on film where you can go much lower).

.... only when a specific condition is met; that the "relative exposure"
or "exposure compensation" remains the same at the different ISOs, and
that the camera/lens combo has the shutter speeds and f-stops to
maintain the EC at all ISOs.

Of course, some of those captures will be blurred, or have less DOF than
desired, or poor, wide-open optics.

This is not a paradigm that most people operate under, but given a
limited amount of available light, the best capture is obtained by
making the photographic compromises between f-stop, shutter speed, vs
the S/N ratio in the sensor (analog), and using the ISO that uses the
biggest part of the range of RAW capture values (the highest ISO) that
doesn't clip desired highlight detail.

The rule of "using the lowest ISO" for maximum capture quality only
works when DOF and shutter speed are not issues (static scene with
tripod and MLU).

>People aren't being
>mislead, it's exactly the same as in the film world.

Not exactly. The film process is analog. The digital process starts
out with an analog exposure, and then digitizes a sub-range of it. Poor
digitization at low ISOs is just as bad, and possibly worse, than
amplified sensor noise at high ISOs.

>Faster films/iso's
>means better low light/hand held photographs, but at the expense of
>grain/noise.

That compromise is not always present with digital.

As I said in another post, if the brightest part of your scene is a
middle grey, and a majority of it, then shooting at +2 EC at 4x the ISO
as a "normal exposure" will result in less noise, not more, if the
camera is truly using 4x the amplification at 4x the ISO. Shooting at
+2 EC at the lowest ISO will be even better, noise-wise, but potentially
impractical in terms of f-stop and shutter speed.

--

<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
John P Sheehy <JPS@no.komm>
><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><
Anonymous
August 10, 2005 1:15:30 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

David J Taylor wrote:

> To get the higher ISOs, for example, and using simple numbers:
>
> - in normal mode, the converter digitises a voltage range of 0..4V to a
> digital range of 0..4000. This means it must be accurate to 0.001V. The
> ADC will work by comparing a signal with a 4V reference value.
>
> - in ISO 3200 mode, the signal range is now just 0..1V. To do this, you
> could
>
> either:
>
> -- quadruple the values from the ADC, turning it into a device digitising
> 0..1V to a digital range of 0..1000, but multiplying each result by 4, so
> that 0..1V returns digital values of 0..4000. The quantisation steps are
> still 0.001V, but as the digital values are quadrupled, the digital levels
> in the image will be in steps of 4; 0, 4, 8, 12, etc.
>
> or:
>
> -- reduce the reference voltage in the converter so that it measures the
> analog voltage against a 1V reference, but still returns values 0..4000.
> The quantisation steps are now 0.0025V. Whilst the analog accuracy of the
> converter may not justify the full 0,0025V steps, digitising this way may
> produce a slightly more accurate result than simply quadrupling the
> values. The digital levels will still be in steps of 1.
>
> Perhaps there is a simple "double the output of the converter" happening
> in cameras where this quantisation is observed? Perhaps both methods are
> used to get two extra ISO steps?

Yes, I owuld guess that it is so. When below, eg, ISO 1600, the ADC is
stepped, and at 1600 and above the sensor readings are left shifted.
There may or may not be an overlapped zone where some gain is in the ADC
and some gain is in shifting left.

Then, there may be some filtering in the ADC as well as (of course)
making the RGB image from the Bayer image that will disgusise (filter,
badly or otherwise) the quantization effects.

Cheers,
Alan


--
-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm
-- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm
-- [SI] gallery & rulz: http://www.pbase.com/shootin
-- e-meil: Remove FreeLunch.
Anonymous
August 10, 2005 1:19:08 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

JPS@no.komm wrote:

> "Quantization" would be clearer, I suppose, even if this is correct. I
> see the word posterized much more than I do quantized, nowadays, and
> assumed the former was more in common usage. People almost always refer
> to an image with too few color levels to represent smooth gradients as
> "posterized".

That comes out of the appearance of the image. Photographers after all.

Quantization noise is further disguised in character, of course, by the
conversion from bayer to RGB.

Cheers,
Alan.

--
-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm
-- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm
-- [SI] gallery & rulz: http://www.pbase.com/shootin
-- e-meil: Remove FreeLunch.
Anonymous
August 10, 2005 7:21:54 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

<JPS@no.komm> wrote in message
news:v07if1dshnpg15b7277mdtd7skqh6kejkm@4ax.com...
SNIP
> The rule of "using the lowest ISO" for maximum capture quality
> only works when DOF and shutter speed are not issues (static
> scene with tripod and MLU).

True, but for me that's automatically solved when I "expose to the
right". If the shutterspeed and aperture are cast in concrete (which
they often aren't), then all that's left is to correct with the ISO
setting (not to influence the exposure meter, I'd probably use Manual
in such a situation) in order to change the amplification on the
analog signal before ADC.

However, if capturing the full scene Dynamic Range is important, I'd
probably choose ISO 100 if I need to get the best sensor DR output,
and adjust the Depth-of-Field / camera shake / subject motion
trade-off.

Bart
Anonymous
August 10, 2005 7:35:46 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

<JPS@no.komm> wrote in message
news:j4dcf1l4hguvog9bsrv0qnnm56k2bc3738@4ax.com...
SNIP
> This is what I'm talking about; this is what I think exposure really
> means (for a given subject intensity, of course). It is often used,
> however, for the relative brightness of an image converted with 0
> exposure adjustment, which, IMO, is more appropriately called
> digitization.

The problem is that you essentially talk about Manual exposure
setting, which changes the ISO setting in an analog signal gain
control. However, in all other (non-manual) exposure settings, the
time x aperture metering settings change in function of the ISO
setting, in addition to the gain control. The latter applies to the
majority of images shot, and thus ISO also changes the amount of
noise.

I'd suggest just using Exposure when using the EV, and EV + gain
control when shooting Manual.

Bart
Anonymous
August 11, 2005 2:58:39 AM

Archived from groups: rec.photo.digital.slr-systems (More info?)

In message <42f9ff9d$0$11066$e4fe514c@news.xs4all.nl>,
"Bart van der Wolf" <bvdwolf@no.spam> wrote:

><JPS@no.komm> wrote in message
>news:v07if1dshnpg15b7277mdtd7skqh6kejkm@4ax.com...
>SNIP
>> The rule of "using the lowest ISO" for maximum capture quality
>> only works when DOF and shutter speed are not issues (static
>> scene with tripod and MLU).
>
>True, but for me that's automatically solved when I "expose to the
>right".

Yes, but do you expose to the right *and* raise the ISO to get the
motion/DOF parameters you need? I think only a minority of people truly
understand that going to a higher ISO is not only sometimes necessary,
but it is also *NOT* necessarily a quality compromise; not unless you
clip the data or lower the absolute sensor exposure witha faster shutter
speed and/or a smaller aperture.

Let's say that you're shooting a black subject against a middle-grey
background with a 100mm lens on a full-frame DSLR, at a distance, and
you want the DOF that is had at f8. The camera's metering tells you
that at ISO 100 and 1/100 and f/8, you are "under-exposed" (or, in my
suggested terms, "under-digitized") by one stop. Common wisdom would
dictate to most people that they need to move to ISO 200. The fact is,
you could move to ISO 800, maybe even 1600, with cleaner, more detailed
results, with the same aperture and shutter speed (for 1600 you might
need to decrease the absolute exposure just a tad to avoid blowing out
the green channel).

Anyone who thinks that what I just wrote is outrageous is clearly
operating in an inefficent exposure/digitization paradigm.

I like to look at things with efficient models, and nothing is clearer
than looking at ISO choice in terms of getting the most output range for
a given absolute sensor exposure.

It isn't ISO per se that causes noise; it is the S/N ratio in the
absolute analog sensor exposure that determines the starting noise, and
ISO has absolutely no effect on that exposure except in how it affects
the camera's metering. It has an effect on how that exposure is
digitized, and at this stage, the higher the ISO is, the less noise
there is in the image, because there is less quantization. Of course,
this assumes analog gains proportional to the ISO numbers.

>If the shutterspeed and aperture are cast in concrete (which
>they often aren't), then all that's left is to correct with the ISO
>setting (not to influence the exposure meter, I'd probably use Manual
>in such a situation) in order to change the amplification on the
>analog signal before ADC.

Too bad this couldn't be done in finer increments than one stop, on most
cameras. You have to vary f-stop and shutter speed by 1/2 or 1/3 stop
to get the in-between levels of digitization.

>However, if capturing the full scene Dynamic Range is important, I'd
>probably choose ISO 100 if I need to get the best sensor DR output,
>and adjust the Depth-of-Field / camera shake / subject motion
>trade-off.

I don't know what camera you have, but I really can't see much of a
difference in image quality between 100 and 200 with my 20D. It seems
to me that 200 has just enough noise to dither the lower bits into
softening the quantization that happens at ISO 100. So, you get
slightly more random noise with 200, and slightly more posterization
effect with 100, and they come up about equal. That one stop can often
go a long way towards a better shutter speed, or get you away from the
poor "wide-open" optics that many lenses have. IS0 400 is noticeably
poorer, and from there up the difference becomes more dramatic with each
doubling of the ISO setting. This, of course, is in reference to shadow
quality. High-key images do not vary as much at the various ISOs.
--

<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
John P Sheehy <JPS@no.komm>
><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><
Anonymous
August 11, 2005 3:15:12 AM

Archived from groups: rec.photo.digital.slr-systems (More info?)

In message <ddculd$13n$1@inews.gazeta.pl>,
Alan Browne <alan.browne@FreeLunchVideotron.ca> wrote:

>Yes, I owuld guess that it is so. When below, eg, ISO 1600, the ADC is
>stepped, and at 1600 and above the sensor readings are left shifted.
>There may or may not be an overlapped zone where some gain is in the ADC
>and some gain is in shifting left.
>
>Then, there may be some filtering in the ADC as well as (of course)
>making the RGB image from the Bayer image that will disgusise (filter,
>badly or otherwise) the quantization effects.

The dynamics of the noise are greater than the dynamics of quantization
at the highest ISOs. The lower ISOs are the ones that show the most
quantization effect.
--

<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
John P Sheehy <JPS@no.komm>
><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><
Anonymous
August 11, 2005 3:23:10 AM

Archived from groups: rec.photo.digital.slr-systems (More info?)

<JPS@no.komm> wrote:

> Let's say that you're shooting a black subject against a middle-grey
> background with a 100mm lens on a full-frame DSLR, at a distance, and
> you want the DOF that is had at f8. The camera's metering tells you
> that at ISO 100 and 1/100 and f/8, you are "under-exposed" (or, in my
> suggested terms, "under-digitized") by one stop. Common wisdom would
> dictate to most people that they need to move to ISO 200. The fact is,
> you could move to ISO 800, maybe even 1600, with cleaner, more detailed
> results, with the same aperture and shutter speed (for 1600 you might
> need to decrease the absolute exposure just a tad to avoid blowing out
> the green channel).
>
> Anyone who thinks that what I just wrote is outrageous is clearly
> operating in an inefficent exposure/digitization paradigm.

It's not outrageous at all -- it just bears no resemblance to any way
I've ever approached photography. I guess it makes sense in the above
situation if you actually believed the camera's light meter, which
was wildly incorrect; if the better exposure was at ISO 800, then the
meter was two full stops off in its judgement, because you weren't
underexposing by one stop at ISO 100, you were underexposing by three
stops. So you're still comparing a proper exposure at ISO 800 to a
two-stops-under exposure at ISO 200, and the moral of the story is
still that it's better to boost ISO than to underexpose.

> Too bad this couldn't be done in finer increments than one stop, on most
> cameras. You have to vary f-stop and shutter speed by 1/2 or 1/3 stop
> to get the in-between levels of digitization.

Do the Canons really make you use full-stop increments??

--
Jeremy | jeremy@exit109.com
Anonymous
August 11, 2005 3:23:54 AM

Archived from groups: rec.photo.digital.slr-systems (More info?)

In message <ddcus6$13n$2@inews.gazeta.pl>,
Alan Browne <alan.browne@FreeLunchVideotron.ca> wrote:

>Quantization noise is further disguised in character, of course, by the
>conversion from bayer to RGB.

Not if you look directly at the RAW data.

Demosaicing and white-balancing certainly hide a multitude of
quantization sins. If a camera had no CFA, and was greyscale, then an
8-bit Tiff with a 2.2-gamma-adjusted output from a greyscale RAW file
would have values of something like 0, 9, 16, 20, 22, etc, in the
deepest shadows, with nothing in-between. The WB and demosaicing create
all kinds of intermediate values that fool a histogram, but doesn't have
quite the shadows that it could, if the camera's sensor "sensed" in a
gamma-adjusted (and already white-balanced) manner.
--

<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
John P Sheehy <JPS@no.komm>
><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><
Anonymous
August 11, 2005 3:47:36 AM

Archived from groups: rec.photo.digital.slr-systems (More info?)

In message <11fl32u6g6ke679@corp.supernews.com>,
Jeremy Nixon <jeremy@exit109.com> wrote:

> <JPS@no.komm> wrote:

>> Let's say that you're shooting a black subject against a middle-grey
>> background with a 100mm lens on a full-frame DSLR, at a distance, and
>> you want the DOF that is had at f8. The camera's metering tells you
>> that at ISO 100 and 1/100 and f/8, you are "under-exposed" (or, in my
>> suggested terms, "under-digitized") by one stop. Common wisdom would
>> dictate to most people that they need to move to ISO 200. The fact is,
>> you could move to ISO 800, maybe even 1600, with cleaner, more detailed
>> results, with the same aperture and shutter speed (for 1600 you might
>> need to decrease the absolute exposure just a tad to avoid blowing out
>> the green channel).

>> Anyone who thinks that what I just wrote is outrageous is clearly
>> operating in an inefficent exposure/digitization paradigm.

>It's not outrageous at all -- it just bears no resemblance to any way
>I've ever approached photography. I guess it makes sense in the above
>situation if you actually believed the camera's light meter, which
>was wildly incorrect; if the better exposure was at ISO 800, then the
>meter was two full stops off in its judgement, because you weren't
>underexposing by one stop at ISO 100, you were underexposing by three
>stops. So you're still comparing a proper exposure at ISO 800 to a
>two-stops-under exposure at ISO 200, and the moral of the story is
>still that it's better to boost ISO than to underexpose.

You misunderstood; there is nothing wrong with the metering in this
scenario. The scene had the approximate reflectivity of a grey card.

>> Too bad this couldn't be done in finer increments than one stop, on most
>> cameras. You have to vary f-stop and shutter speed by 1/2 or 1/3 stop
>> to get the in-between levels of digitization.

>Do the Canons really make you use full-stop increments??

For ISO? Yes, the ISOs are 100, 200, 400, 800, 1600, 3200 On the
non-1-series cameras.
--

<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
John P Sheehy <JPS@no.komm>
><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><
Anonymous
August 11, 2005 5:11:30 AM

Archived from groups: rec.photo.digital.slr-systems (More info?)

<JPS@no.komm> wrote:

> You misunderstood; there is nothing wrong with the metering in this
> scenario. The scene had the approximate reflectivity of a grey card.

No, there was; if the meter indicates one stop underexposure, but you can
increase three stops and end up with a proper exposure, then the meter was
actually off by two stops. You're comparing "trust the meter" at ISO 200
with "don't trust the meter" at ISO 800, and that's not a useful
comparison, because if you trusted the meter in the first situation
you'd trust it in the second and still underexpose by two stops, but if
you weren't going to trust the meter in the first place (and thus be
willing to go up two more stops from what it said at ISO 800) then you'd
have been willing to do so at ISO 200, too, if you could -- leaving the
choices being either underexpose at 200 or expose properly at 800.

>> Do the Canons really make you use full-stop increments??
>
> For ISO? Yes, the ISOs are 100, 200, 400, 800, 1600, 3200 On the
> non-1-series cameras.

Ah, so it's one of those "held back" features. Annoying how they use
(seemingly) simple features to convince you to spend more money.

--
Jeremy | jeremy@exit109.com
Anonymous
August 11, 2005 12:50:43 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

In message <11fl9e2f2hq0qe4@corp.supernews.com>,
Jeremy Nixon <jeremy@exit109.com> wrote:

> <JPS@no.komm> wrote:
>
>> You misunderstood; there is nothing wrong with the metering in this
>> scenario. The scene had the approximate reflectivity of a grey card.
>
>No, there was; if the meter indicates one stop underexposure, but you can
>increase three stops and end up with a proper exposure,

What is "proper exposure"?

>then the meter was
>actually off by two stops.

No, the meter is dumb, and it puts the mid-key scene at middle grey,
which is not *optimum*, unless your goal is to use a JPEG as-is, or
print directly from the camera. In terms of RAW capture, it is a
relatively poor digitization and/or exposure.

>You're comparing "trust the meter" at ISO 200
>with "don't trust the meter" at ISO 800, and that's not a useful
>comparison, because if you trusted the meter in the first situation
>you'd trust it in the second and still underexpose by two stops, but if
>you weren't going to trust the meter in the first place (and thus be
>willing to go up two more stops from what it said at ISO 800) then you'd
>have been willing to do so at ISO 200, too, if you could -- leaving the
>choices being either underexpose at 200 or expose properly at 800.

I clearly stated that we were a stop under because of *NEEDED* f-stop
and shutter speed! You're going to blur the image now?

Let me try another way. Let's pretend the camera is greyscale (no CFA),
to simplify matters. The camera meters a grey card (or the ~18% grey
scene) as if to wind up with an average RAW value of 400 (fairly
typical) out of about 4000 possible levels. At ISO 100, you fall a stop
short of that with your needed f-stop and shutter speed, so the RAW
output would be 200. Using the same aperture and shutter speed (as
needed), with ISO 800, the average now is a RAW value of 1600. If you
used ISO 1600, the average RAW value would be 3200, and you might have a
few brighter spots clipping past 4095.

--

<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
John P Sheehy <JPS@no.komm>
><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><
Anonymous
August 11, 2005 11:05:39 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

<JPS@no.komm> wrote:
> Jeremy Nixon <jeremy@exit109.com> wrote:
>
>> No, there was; if the meter indicates one stop underexposure, but you can
>> increase three stops and end up with a proper exposure,
>
> What is "proper exposure"?

The exposure you want is the proper exposure. In this case, +2 from the
meter reading.

>> then the meter was actually off by two stops.
>
> No, the meter is dumb, and it puts the mid-key scene at middle grey,
> which is not *optimum*, unless your goal is to use a JPEG as-is, or
> print directly from the camera. In terms of RAW capture, it is a
> relatively poor digitization and/or exposure.

Right. The meter is off by two stops from the exposure you want.

>> You're comparing "trust the meter" at ISO 200
>> with "don't trust the meter" at ISO 800, and that's not a useful
>> comparison, because if you trusted the meter in the first situation
>> you'd trust it in the second and still underexpose by two stops, but if
>> you weren't going to trust the meter in the first place (and thus be
>> willing to go up two more stops from what it said at ISO 800) then you'd
>> have been willing to do so at ISO 200, too, if you could -- leaving the
>> choices being either underexpose at 200 or expose properly at 800.
>
> I clearly stated that we were a stop under because of *NEEDED* f-stop
> and shutter speed! You're going to blur the image now?

No. Using the needed f-stop and shutter speed, at ISO 200 you are
underexposing, and at 800 you are not. You're suggesting that at
ISO 200 you are blindly trusting the meter, but once you go to 800
you suddenly aren't and are willing to use +2 EC from what it's
telling you to get a good exposure.

In real life, if someone is willing to use +2 EC to get a good exposure
at ISO 800, then they were willing to do so at ISO 200, and the only
reason they didn't is because the shutter speed would end up too low.
So, the conclusion is that there isn't enough light for ISO 200, and
the shot would be underexposed. Since it's better to boost the ISO
than to underexpose, you boost the ISO.

The properly exposed shot at ISO 800 will be of better technical quality
than the underexposed one at 200. However, the properly exposed shot
at 800 will have *more* noise than the properly exposed shot at ISO 200;
it's just that you couldn't get the shot at 200.

The usual thought process here, from the photographer's perspective,
would be: good exposure is at +2 EC. I can't get there with shutter
speed because it'll be too slow to hand-hold; I can't get there with
aperture because I need the depth of field; so I get there with ISO.

> Let me try another way. Let's pretend the camera is greyscale (no CFA),
> to simplify matters. The camera meters a grey card (or the ~18% grey
> scene) as if to wind up with an average RAW value of 400 (fairly
> typical) out of about 4000 possible levels. At ISO 100, you fall a stop
> short of that with your needed f-stop and shutter speed, so the RAW
> output would be 200. Using the same aperture and shutter speed (as
> needed), with ISO 800, the average now is a RAW value of 1600. If you
> used ISO 1600, the average RAW value would be 3200, and you might have a
> few brighter spots clipping past 4095.

Right. At ISO 100, you're drastically underexposing. At 200, you're
still underexposing. At 800, you're not. At 1600, you're overexposing,
unless you intended for those bright spots to clip (spectral reflections,
point light sources, etc).

--
Jeremy | jeremy@exit109.com
!