Question about f numbers ?

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

Is it always true that a larger lens will have a lower f number ?

If so, can someone explain to we why fujifilm 2800zoom has
a bigger lens, about 2.5cm, than the olympus c170, which is
just 1.3cm, and yet both cameras are the same f number, i.e.
2.8 ?

tia

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

news () wrote in news:428e1dd1$1_1@mk-nntp-2.news.uk.tiscali.com:

> Is it always true that a larger lens will have a lower f number ?

No.

> If so, can someone explain to we why fujifilm 2800zoom has
> a bigger lens, about 2.5cm, than the olympus c170, which is
> just 1.3cm, and yet both cameras are the same f number, i.e.
> 2.8 ?

The "f-number" is determined by dividing the focal length of the lens
by the effective diameter of the aperture; the physical size of the
lens assembly doesn't enter into the equation at all.

--
Bert Hyman | St. Paul, MN | bert@iphouse.com

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

<news> wrote in message news:428e29f1$1_2@mk-nntp-2.news.uk.tiscali.com...
>> > Is it always true that a larger lens will have a lower f number ?
>>
>> No.
>>
>> > If so, can someone explain to we why fujifilm 2800zoom has
>> > a bigger lens, about 2.5cm, than the olympus c170, which is
>> > just 1.3cm, and yet both cameras are the same f number, i.e.
>> > 2.8 ?
>>
>> The "f-number" is determined by dividing the focal length of the lens
>> by the effective diameter of the aperture; the physical size of the
>> lens assembly doesn't enter into the equation at all.
>
> Well, it must be part of the "brightness factor" of the lens. People
> often refer to "bright" lenses as "low-f number" lenses.
>
> Now, if two cameras have the same f number, they must be
> getting the same amount of exposure,
> the question is if my olympus camera, which has a small lens,
> can receive the same exposure as the bigger fujifilm 2800zoom,
> what is the purpose for using a bigger lense?
>
>
Also, larger sensors will need longer focal length lenses for the same angle
of view, thus the lens will be larger. I don't know about the two cameras
being compared. If one had a longer zoom (tele) the lens could be larger in
this case as well.
John

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

news () wrote in news:428e29f1$1_2@mk-nntp-2.news.uk.tiscali.com:

>> > Is it always true that a larger lens will have a lower f number
>> > ?
>>
>> No.
>>
>> > If so, can someone explain to we why fujifilm 2800zoom has
>> > a bigger lens, about 2.5cm, than the olympus c170, which is
>> > just 1.3cm, and yet both cameras are the same f number, i.e.
>> > 2.8 ?
>>
>> The "f-number" is determined by dividing the focal length of the
>> lens by the effective diameter of the aperture; the physical size
>> of the lens assembly doesn't enter into the equation at all.
>
> Well, it must be part of the "brightness factor" of the lens.
> People often refer to "bright" lenses as "low-f number" lenses.
>
> Now, if two cameras have the same f number, they must be
> getting the same amount of exposure,
> the question is if my olympus camera, which has a small lens,
> can receive the same exposure as the bigger fujifilm 2800zoom,
> what is the purpose for using a bigger lense?

I'm not really sure what you mean by "small" or "bigger" lens.

Are you talking about focal length or physical size?

Simple physical size is more a matter of packaging, engineering or
even esthetic concerns, rather than optics.

Focal length determines image scale on the film or sensor. For
objects of the same size and distance from the camera, a larger
(longer) focal length lens will produce a larger image.

--
Bert Hyman | St. Paul, MN | bert@iphouse.com

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

> I'm not really sure what you mean by "small" or "bigger" lens.

When I say a bigger lens, I mean that the diameter of the round bit
of glass where the light goes into is larger.

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

> > Is it always true that a larger lens will have a lower f number ?
>
> Nope. There are two kinds of 'f number.'
>
> On some lenses you will see something like 'f=50mm.' This means the
> focal length (f) is 50mm, so you'd call it a 50mm lens.

No, what I mean is f numbers, such as 1.8, 2.0, 2.8, 4.0, 5.6, 8, 11

>
> The aperture setting is a ratio of the focal length (f), so f/2 (an
> f-stop of 2) is half of the focal length. In the example above, since
> f=50, we get 50/2, or 25mm.

I don't really know about these mm values, but really this is not what
I am trying to ask about

>
> These numbers get mixed around in strange ways, too, all over the place.
> For instance, I just did an ebay search for lenses, and found a lens
> that says this: F2/85mm. Obviously they mean f=85mm, and the widest
> aperture is f/2.

Dunno, maybe, but this is not what I was asking...

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

In news:428e5f9d$1_2@mk-nntp-2.news.uk.tiscali.com <news> wrote:

>> I'm not really sure what you mean by "small" or "bigger" lens.
>
> When I say a bigger lens, I mean that the diameter of the round bit
> of glass where the light goes into is larger.

OK; for a given focal length, a larger diameter lens suggests, but isn't a
certain indication of a faster lens.

The f-number is determined by the ->apparent size of the aperture, which
is the aperture size when viewed through the front element. For zoom
lenses (which most cameras have these days) that will vary significantly
as you chage focal length. This is complicated by the fact that with
automatic cameras, you can't actually see the stopped-down aperture except
for the brief instant when the exposure is actually being made.

At any rate, in summary, to compare the "speed" of a lens, you have to
take into account both the focal length and the f-number. For zoom lenses,
you have to consider the maximum and minimum focal length and the f-number
at both extremes, if the manufacturer publishes that information.

--
Bert Hyman St. Paul, MN bert@iphouse.com

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

news wrote:

> Is it always true that a larger lens will have a lower f number ?
>
> If so, can someone explain to we why fujifilm 2800zoom has
> a bigger lens, about 2.5cm, than the olympus c170, which is
> just 1.3cm, and yet both cameras are the same f number, i.e.
> 2.8 ?

I'll take a stab at it. The diameter of the front element can be
different for a number of reasons.

One which hasn't been mentioned is focal length. A really wide/short
fish eye type lens is going to tend to be wider though it varies
depending on the design.

Another factor is a large maximum aperture (smallest f/stop) tends to
make the front element larger diameter though again it depends on the
design. The aperture is the diameter of the iris opening inside but the
distance between each piece of glass and the focal length determines the
diameter of the front element. Large aperture "bright" lenses are called
"fast" lenses because the larger opening lets more light in allowing
faster/shorter shutter speeds. Nobody calls them "bright", that would be
too easy!

: - )

The focal length, I don't quite understand but I do know that can vary a
lot depending on the design. In simplest terms it's the length of the
lens but they use sometimes more than a dozen pieces of glass inside,
each a different convex or concave shape, shperical or aspherical, etc
that work in combination to control things like flat field focus and
chromatic abberation which is mis-matching of different colors due to
their different wavelengths and of course making a zoom lens is much
more complicated than a fixed focal length lens. These are some of the
design differences that could lead to a different sized front glass
element diameter.


--
Paul Furman
http://www.edgehill.net/1
san francisco native plants

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

> >> I'm not really sure what you mean by "small" or "bigger" lens.
> >
> > When I say a bigger lens, I mean that the diameter of the round bit
> > of glass where the light goes into is larger.
>
> OK; for a given focal length, a larger diameter lens suggests, but isn't a
> certain indication of a faster lens.
>
> The f-number is determined by the ->apparent size of the aperture, which
> is the aperture size when viewed through the front element. For zoom
> lenses (which most cameras have these days) that will vary significantly
> as you chage focal length. This is complicated by the fact that with
> automatic cameras, you can't actually see the stopped-down aperture except
> for the brief instant when the exposure is actually being made.
>
> At any rate, in summary, to compare the "speed" of a lens, you have to
> take into account both the focal length and the f-number. For zoom lenses,
> you have to consider the maximum and minimum focal length and the f-number
> at both extremes, if the manufacturer publishes that information.

Why would I need to consider a zoom situation if I am asking about a
non-zoom
comparison. The camera may be a zoom one, but I can always use it without
a zoom, in which case it behaves as if it does not posses a zoom, so I don't
believe the zoom has any relevance to this discussion.

>
> --
> Bert Hyman St. Paul, MN bert@iphouse.com

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

<news> writes:

>In that case my question is if a camera (digital) has an f stop of 2.8, and
>another camera has the same f stop, do they both receive the same
>amount of light into the CCD?

Almost, maybe.

Two cameras that have lenses with the same f/number would have the
same *light intensity per unit area*, if there was no light loss in the
glass.

Now, if camera A has a sensor that is 2 X larger than camera B, the
sensor in A has 4 times the area of the sensor in B. To cover the same
field of view, camera A's lens focal length needs to be twice that of
camera B. In order for the lens f/number to remain the same, the lens
entrance pupil for camera A needs to be twice the diameter or 4 times
the area as the one for camera B.

So camera A's lens collects 4 X as many photons per unit time, but its
focal length is such that it spreads those photons over 4 X as much area
in the image plane, so the photon flux per unit area is the same

Dave

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

<news> wrote in message news:428f1a8b$1_3@mk-nntp-2.news.uk.tiscali.com...
>> Is it always true that a larger lens will have a lower f number ?
>>
>> If so, can someone explain to we why fujifilm 2800zoom has
>> a bigger lens, about 2.5cm, than the olympus c170, which is
>> just 1.3cm, and yet both cameras are the same f number, i.e.
>> 2.8 ?
>
> I think I see now, see if you agree
>
> f stop/number does't imply aperture size, because the the focal
> lense can be different between two camera's even if zoom is
> not used/engaged.
>
> However, intuitively, a bigger front element will let more light in and
> will
> therefore be faster, regardless of the f number (assuming equal CCD's)
>
> In my case, the fujifilm has the same f as the olympus i.e. 2.8, hence the
> focal length should be longer than the olympus, which means the light
> intensity into the aperture will be greater than the oympus because the
> lense fill focus the light into a stronger intensity.
>
>
>
Rubbish!!!!!

If two lenses have the same f number, the intesity of light at the sensor or
film will be the same.

The above is true irrespective of sensor size or focal length or size of the
front piece of glass, or almost any other factor, apart from adding filters
in front of the lens.

If this were NOT TRUE, it would be virtually impossible to use a camera with
interchangeable lenses.

The reply from Dave Martindale explained it exactly.

The Focal length of the lens is taken into account in calculating the f
number.

The actual aperture size (pupil), is taken into account when the f number
is being calculated.

The size of the front piece of glass, may be different from the aperture
size of that lens, and does not have any direct bearing on the f number
calculation.

Once the f number has been calculated the focal length and pupil size, have
no more relevance to the amount of light being transmitted.

The f number is all that matters.

There will, of course, be some inaccuracies in these calculations, but any
differences will be very slight, and we, the photographers, will never
notice.

A "Bright"or "Fast" lens will have a lower f number than a "Slower" or
"Less Bright" lens.

Roy G

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

> You really seem to be going out of your way to annoy the people who are
> trying to help you and to take as little information as possible out of
> this discussion.

If you are annoyed than that is probably because you are still having
difficulty to understand what I am asking or you are frustrated that
suddenly you realise that you have not got the answer either. In
any case, your lack of knowledge is what is leading to your
frustration, I am perfectly allowed to ask in straight forward English.

Now, can you answer this (if you can't you know why you are
frustrated)

There are two digital cameras
Both are slim compact size
Both do not have zoom
Both are from recent times, lets say 2004
Both are equivalent of 38mm in the old era of 35mm
Both have an f stop number of 2.8
One has a front element (lens) twice as big as the other

Is this scenario possible?

Now answer this one:

You have 2 digital camers
Both are slim compact size without zoom from 2004
Both are 38mm equivalent of the 35mm era
Both have an f stop number of 2.8
Both cameras have the same max exposure duration for night photography.

1. Can you conclude from that they both cameras will have the
same lense ?

2. Can you conclude from that that both cameras will get the
same brightness on night photography assuming the light
is insufficient and therefore max exposure time will automatically
be selected.

3. If you see another camera as described above, but with f=5.6
instead of 2.8, what does that tell you (if anything) on the brightness
of a night picture when maximum exposure time is selected, will
it be greater equal to or less than in (2) above

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

Dave Martindale wrote:

> <news> writes:
>
>
>>In that case my question is if a camera (digital) has an f stop of 2.8, and
>>another camera has the same f stop, do they both receive the same
>>amount of light into the CCD?
>
>
> Almost, maybe.
>
> Two cameras that have lenses with the same f/number would have the
> same *light intensity per unit area*, if there was no light loss in the
> glass.
>
> Now, if camera A has a sensor that is 2 X larger than camera B, the
> sensor in A has 4 times the area of the sensor in B. To cover the same
> field of view, camera A's lens focal length needs to be twice that of
> camera B. In order for the lens f/number to remain the same, the lens
> entrance pupil for camera A needs to be twice the diameter or 4 times
> the area as the one for camera B.
>
> So camera A's lens collects 4 X as many photons per unit time, but its
> focal length is such that it spreads those photons over 4 X as much area
> in the image plane, so the photon flux per unit area is the same
>
> Dave


Heh, and if you put a polarizing filter on the front or change the ISO
speed rating, things get wonky as the exposure needed for the same
shooting conditions (just trying to keep it confusing <grin> ). My point
there is that the same f/stop in those situations would have a different
amount of light or different effect on exposure.

--
Paul Furman
http://www.edgehill.net/1
san francisco native plants

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

<news> wrote:

>> You really seem to be going out of your way to annoy the people who
>> are trying to help you and to take as little information as
>> possible out of this discussion.
>
> If you are annoyed than that is probably because you are still
> having difficulty to understand what I am asking or you are
> frustrated that suddenly you realise that you have not got the
> answer either. In any case, your lack of knowledge is what is
> leading to your frustration, I am perfectly allowed to ask in
> straight forward English.
The problem is YOUR lack of comprehension.
>
> Now, can you answer this (if you can't you know why you are
> frustrated)
He is frustrated by your attitude.
>
> There are two digital cameras
> Both are slim compact size
> Both do not have zoom
> Both are from recent times, lets say 2004
> Both are equivalent of 38mm in the old era of 35mm
> Both have an f stop number of 2.8
> One has a front element (lens) twice as big as the other
>
> Is this scenario possible?
Yes it is possible although unlikely.
>
> Now answer this one:
>
> You have 2 digital camers
> Both are slim compact size without zoom from 2004
> Both are 38mm equivalent of the 35mm era
> Both have an f stop number of 2.8
> Both cameras have the same max exposure duration for night
> photography.
>
> 1. Can you conclude from that they both cameras will have the
> same lense ?
Only if the cameras both have interchangeable lenses with the same
fitting and you change the lens from one camera to another.
>
> 2. Can you conclude from that that both cameras will get the
> same brightness on night photography assuming the light
> is insufficient and therefore max exposure time will automatically
> be selected.
This paragraph is virtually meaningless.
>
> 3. If you see another camera as described above, but with f=5.6
> instead of 2.8, what does that tell you (if anything) on the
> brightness of a night picture when maximum exposure time is
> selected, will it be greater equal to or less than in (2) above
Use the brain that god gave you in conjunction with the info. already
posted in this thread.
--
neil
delete delete to reply

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

news wrote:
>>
>>At any rate, in summary, to compare the "speed" of a lens, you have to
>>take into account both the focal length and the f-number. For zoom lenses,
>>you have to consider the maximum and minimum focal length and the f-number
>>at both extremes, if the manufacturer publishes that information.
>
>
> Why would I need to consider a zoom situation if I am asking about a
> non-zoom
> comparison. The camera may be a zoom one, but I can always use it without
> a zoom, in which case it behaves as if it does not posses a zoom, so I don't
> believe the zoom has any relevance to this discussion.

Zoom lenses often have a smaller maximum aperture at the long end like
f/3.5 to 5.6 is common. So it depends whether you are talking about the
long or short end ans the front element may be designed for the other
end of the focal range, or another reason like macro ability perhaps.
Also zoom lenses have to go through more contortions and need more
pieces of glass & room to move them and theredore need to make more
corrections so it's really a very complicated situation. No simple rules
to apply.


--
Paul Furman
http://www.edgehill.net/1
san francisco native plants

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

news wrote:

>>>Is it always true that a larger lens will have a lower f number ?
>>
>>Nope. There are two kinds of 'f number.'
>>
>>On some lenses you will see something like 'f=50mm.' This means the
>>focal length (f) is 50mm, so you'd call it a 50mm lens.
>
>
> No, what I mean is f numbers, such as 1.8, 2.0, 2.8, 4.0, 5.6, 8, 11
>
>
>>The aperture setting is a ratio of the focal length (f), so f/2 (an
>>f-stop of 2) is half of the focal length. In the example above, since
>>f=50, we get 50/2, or 25mm.
>
>
> I don't really know about these mm values, but really this is not what
> I am trying to ask about
>
>
>>These numbers get mixed around in strange ways, too, all over the place.
>>For instance, I just did an ebay search for lenses, and found a lens
>>that says this: F2/85mm. Obviously they mean f=85mm, and the widest
>>aperture is f/2.
>
>
> Dunno, maybe, but this is not what I was asking...

The thing is, the two are related, though not directly to your question.
I'll try to go through this step by step... stop me on any part that
doesn't make sense.

First of all, the f-stop (such as f/4) describes the width of the
aperture opening, period, and thus the amount of light it lets through.
But there's an important distinction to make: the "f-number" or
"f-rating" listed on a lens generally refers to the MAXIMUM aperture
opening of that lens - when shooting, many better cameras can reduce the
aperture to control the amount of light on a shot-by-shot basis.

The letter "f" itself refers to the focal length of the lens.
At f/4, the diameter of the aperture is 1/4 the focal length of the
lens; at f/8, the diameter of the aperture is 1/8 the focal length, and
so on.

On a lens with a 50mm focal length, the aperture would be 12.5mm in
diameter at f/4 (50/4 = 12.5).

On a lens with a 30mm focal length, the aperture would be 7.5mm in
diameter at f/4.

In both cases, the same amount of light is getting through. At 30mm,
the lens has a smaller aperture, but a wider field of view, and thus
collects more light than the 50mm lens. The smaller aperture allows
less light through, so at f/4, both lenses are allowing the same amount
of light to reach the film or sensor.

Fuji doesn't list the actual focal lengths of the 2800, only the
equivalent in 35mm, which they give as 30-228mm; the Olympus is a fixed
focal length of 35mm. Right away, the Fuji's lens will be physically
bigger to accomodate the zoom mechanicals and the extended focal length.

Using Fuji's numbers just for the sake of calculation, at 228mm, an f/4
aperture on the Fuji would have to be 57mm in diameter, so the lens
structure itself would have to be AT LEAST that wide at its narrowest
point.

The Olympus, at 35mm, would only need a maximum aperture of 17.5mm to
allow f/2, so it can be a much smaller lens structure.


---
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Virus Database (VPS): 0520-4, 05/20/2005
Tested on: 5/21/2005 12:51:53 PM
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Archived from groups: rec.photo.digital (More info?)

On Sat, 21 May 2005 15:57:34 +0100, <news> wrote:

>> >> Is it always true that a larger lens will have a lower f number ?
>> >>
>> >> If so, can someone explain to we why fujifilm 2800zoom has
>> >> a bigger lens, about 2.5cm, than the olympus c170, which is
>> >> just 1.3cm, and yet both cameras are the same f number, i.e.
>> >> 2.8 ?
>> >
>> > I think I see now, see if you agree
>> >
>> > f stop/number does't imply aperture size, because the the focal
>> > lense can be different between two camera's even if zoom is
>> > not used/engaged.
>> >
>> > However, intuitively, a bigger front element will let more light in and
>> > will
>> > therefore be faster, regardless of the f number (assuming equal CCD's)
>> >
>> > In my case, the fujifilm has the same f as the olympus i.e. 2.8, hence
>the
>> > focal length should be longer than the olympus, which means the light
>> > intensity into the aperture will be greater than the oympus because the
>> > lense fill focus the light into a stronger intensity.
>> >
>> >
>> >
>> Rubbish!!!!!
>>
>> If two lenses have the same f number, the intesity of light at the sensor
>or
>> film will be the same.
>>
>> The above is true irrespective of sensor size or focal length or size of
>the
>> front piece of glass, or almost any other factor, apart from adding
>filters
>> in front of the lens.
>>
>> If this were NOT TRUE, it would be virtually impossible to use a camera
>with
>> interchangeable lenses.
>>
>> The reply from Dave Martindale explained it exactly.
>>
>> The Focal length of the lens is taken into account in calculating the f
>> number.
>>
>> The actual aperture size (pupil), is taken into account when the f number
>> is being calculated.
>>
>> The size of the front piece of glass, may be different from the aperture
>> size of that lens, and does not have any direct bearing on the f number
>> calculation.
>>
>> Once the f number has been calculated the focal length and pupil size,
>have
>> no more relevance to the amount of light being transmitted.
>>
>> The f number is all that matters.
>>
>> There will, of course, be some inaccuracies in these calculations, but any
>> differences will be very slight, and we, the photographers, will never
>> notice.
>>
>> A "Bright"or "Fast" lens will have a lower f number than a "Slower" or
>> "Less Bright" lens.
>
>In that case, this is good because it actually makes sense, otherwise why
>would manufacturers bother telling us what the f stop number is. What
>I find difficult to understand is if two cameras have the same f stop
>number, would it make any sense to you that one of them would have
>a front element (lens?) which is twice as big ? This would suggest
>to me that the brightness of a lens is dependent also on the physical size
>of the front glass/lens, otherwise why would manufacturers use bigger
>lenses, they are probably more expesive to produce, and certainly make
>the camera more bulky and less user friendly.
>
>
>
>>
>> Roy G
>>
>>
>
Right. The penny isn't dropping so lets try something else. The
f-stop is just a photographers 'unit of measurement' so lets forget it
for the time being. Let's also forget about lens focal length.

Imagine two digital cameras - A & B. Camera A has a CCD the size of a
postage stamp and Camera B has a CCD the size of a football pitch.

To get the same amount of light per unit area on each CCD. How big
does the lens have to be on camera A and how big does it have to be on
camera B?

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

On Sat, 21 May 2005 21:20:17 +0100, <news> wrote:

> I am starting to "see the picture now" ...
> Would you say then, when we use the zoom camera (fuji) but do not
> apply the zoom, i.e. on wide settings, we are actually not benefitting
> at all from the bigger front lens, because the bigger lens only makes
> a difference when the zoom is used ?
>
> Hopefully the answer to this will be yes, and then I am satisfied that
> I do understand ...

I'd say yes, more or less. If Fuji had designed your camera
with a lens having half the length of yours, ie, 38-114mm, I'm sure
the front element lens would have been far smaller. But it's a
complex issue, since zoom lens designs are also complex. Whether
all of the front element glass is used at the widest aperture at
wide ang . . . uh, let me rephrase that. It *may* be possible that
when the lens is zoomed to the wide end, and the maximum aperture is
used, some of the light impinging on the sensor *may* have passed
through the far edges of the lens. Whether that happens or not may
depend on the design of the lens. With lenses, what appears to be
"common sense" may not always lead you to the right conclusions.
For instance, my Fuji zoom lens (10x, 37 to 370 35mm equivalent) has
a fairly small front element, a bit less than an inch in diameter.
You'd think that Fuji's wide angle adapter lens for it would also
have a fairly small front lens element. But it's actually much
larger. With the extension tube needed to attach it to the camera,
most people would think it was a really a long telephoto lens with a
really large, heavy piece of glass on the end of it.

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

<news> wrote:

I call troll.

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

<news> writes:

>There are two digital cameras
>Both are slim compact size
>Both do not have zoom
>Both are from recent times, lets say 2004
>Both are equivalent of 38mm in the old era of 35mm
>Both have an f stop number of 2.8
>One has a front element (lens) twice as big as the other

>Is this scenario possible?

Of course. Just because both are compact, this doesn't mean that the
sensors are the same size. If one sensor is twice the size of the
other, then the true focal length of its lens is necessarily double in
order to be "equivalent to 38 mm". Double the focal length and you
necessarily double the aperture needed to give the same f/number.

The other factor is that it's common for the front element in a wide
angle lens to be quite a bit larger than you'd expect from a calculation
of the entrance pupil size from f/number and focal length. The reason
is that the lens has to let through light from the corners of the scene,
not just the centre, and the front element needs to be larger to avoid
vignetting. But *how much* larger depends on how deep into the lens the
entrance pupil is buried, which depends on lens design even when focal
length and f/number remain the same.

Do the actual focal lengths (not the 35 equivalent focal lengths) of the
two cameras match?

>Now answer this one:

>You have 2 digital camers
>Both are slim compact size without zoom from 2004
>Both are 38mm equivalent of the 35mm era
>Both have an f stop number of 2.8
>Both cameras have the same max exposure duration for night photography.

>1. Can you conclude from that they both cameras will have the
>same lense ?

Of course not. What kind of question is this? Unless they are from the
same manufacturer, chances are the lens is a different design. There
are lots of ways to make a f/2.8 lens.

>2. Can you conclude from that that both cameras will get the
>same brightness on night photography assuming the light
>is insufficient and therefore max exposure time will automatically
>be selected.

Do they have the same ISO rating? If not, the one with the higher ISO
will probably get a better exposed picture. Also, many cameras will
*not* automatically select the maximum exposure duration unless you put
them into some sort of "long shutter" mode.

>3. If you see another camera as described above, but with f=5.6
>instead of 2.8, what does that tell you (if anything) on the brightness
>of a night picture when maximum exposure time is selected, will
>it be greater equal to or less than in (2) above

Where is this going? You've piled up a lot of if's by this point. If
the sensors are the same ISO sensitivity, and the exposure times are the
same, and the aperture difference is the *only* difference, then the
camera with the f/2.8 lens with get 4 times as much light on the focal
plane as the camera with the f/5.6 lens, and take a better picture if
there isn't enough light for either camera.

Dave

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

In article <428e1dd1$1_1@mk-nntp-2.news.uk.tiscali.com>, <news> says...
> Is it always true that a larger lens will have a lower f number ?
>
> If so, can someone explain to we why fujifilm 2800zoom has
> a bigger lens, about 2.5cm, than the olympus c170, which is
> just 1.3cm, and yet both cameras are the same f number, i.e.
> 2.8 ?
>
> tia

As others have explained the focal length of the lens needs to be
considered. The focal length is the distance from the lens to the
surface of the film or sensor. For a given lens size the longer the
focal length the higher the f number.

The camera controls how much light hits the sensor in two ways. One way
is the shutter speed which controls the amount of time the film or
sensor is exposed to light. Other way is to control aperture or f-stop
which opens and closes much the same way the iris in your eyes work.

Depending on your camera you can control either the shutter or aperture
or both. This gives you the ability to control how your camera takes the
picture. A fast shutter speed allows you to stop motion. The aperture
allows you to control the septh of field. By that I mean how much of the
picture is in focus. The higher the f number, the more of the scene is
in focus.

I hope this answers your question. If not post s follow up and me or
someone else will attempt to clear up any confusion you may have.

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