# Question about f numbers ?

Tags:
Last response: in Digital Camera
Share

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

Anonymous

<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.

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.

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.

<news> wrote in message 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 ?
Because the diameter of the diaphragm determines the f stop.
Jim
Anonymous

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
Related ressources

> > 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?
Anonymous

<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
Anonymous

On Fri, 20 May 2005 19:16:40 +0100, <news> wrote:

>> > 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?

Your mixing a lot of stuff up here, including terminology.

'Bright' lenses are ones that are *capable* of 'opening up' to a wide
aperture. It's measured in f/stops and f1.2 for example would be a
good bright lens.... Both the cameras you mention have f2.8 lenses.

The fujifilm is has an optical zoom of 30mm-228mm (measured as 35mm
equivalent). Normal focus 2.6ft onwards, macro focus to 3.9 inches.

The Olympus has no optical zoom capability. It's a fixed 35mm lens.
This is a HUGE quality problem if you have to use the digital zoom
function. At full digital zoom, your 4megapixels become 1megapixel. So
the 2.8 megapixel fuji with it's nice optical zoom wins bigtime.

Back to aperture, you'll usually see on a zoom lens that the brightest
aperture actually changes depending on the zoom setting. I've neither
the time nor patience to compare these two, other than to let you know
that everything isn't always just a simple number.

As for physically bigger lenses, they cost more to make and they
generally give better quality images - partly because quality hasn't
been traded for size, partly because they gather more light.

These two cameras are completely different beasts.

--
Owamanga!
http://www.pbase.com/owamanga
Anonymous

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
Anonymous

<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 ?
>
> tia
The 'f no' is a ratio. It is the nominal aperture divided into the
nominal focal length so that for 2 apertures of the same size for 2
differing focal length lenses the one for the shorter focal length
will have the smaller f. no. making it the 'faster' lens.

I hope that you can understand my rubbish.
--
neil

> 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.
Anonymous

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
Anonymous

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

> > 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

>
> 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...
Anonymous

http://www.dpreview.com/learn/?/glossary/Optical/

--
John

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
>
>>
>> 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...
Anonymous

<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

Those aren't 'f numbers,' they're aperture settings, or stops.

> > 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 am

You asked if larger lenses will always have a lower 'f number.' The
answer is no, and I explained why.

> > 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...

True. I added that. You're welcome.

This is simple, the olympus is a fixed focal length lens (digital zoom), the
other is the 35mm equiv of a 38-228mm optical zoom. To get the same Fstop
on the short end being a tele, the fuji needs a larger front element. Also
could easily be the fuji has a larger sensor as well.

--

Stacey

> >> 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
Anonymous

<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
Anonymous

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
Anonymous

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

> This is simple, the olympus is a fixed focal length lens (digital zoom),
the
> other is the 35mm equiv of a 38-228mm optical zoom. To get the same Fstop
> on the short end being a tele, the fuji needs a larger front element. Also

Again, the fuji is a zoom camera, but I am asking regarding the situation
where the zoom facility is actually not used, this is what enables me
to make a comparison between apples and apples, rather than between
apples and oranges. So if it helps at all, consider the situation where the
zoom does not factor into the picture, then, try to answer what I asked.

> could easily be the fuji has a larger sensor as well.

The CCD of the fujifilm 2800zoom is 1/2.7" , I don't know, do we need
to calculate it? 1/2.7=0.3703704" by my calculator

For the Olympus C-170 it is 1/2.5", which gives me 0.4"

0.4" > 0.37" hence it seems the Olympus is actually larger, in any case
there is only 0.03" in it, so they are pritty much the same, yet the front
element of the fuji is twice as big???? and both of them have the same f
stop number, which is 2.8

>
> --
>
> Stacey

> 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.

<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
Anonymous

In news:428f098d\$1_3@mk-nntp-2.news.uk.tiscali.com <news> wrote:

> 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.

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.

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

> 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?

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

> >> 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
>
>
Anonymous

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

> 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.

The problem you're having is due to not comparing two similar
types of lenses. Your assumption is valid for comparing two fixed
length lenses. If they have the same focal length and the same "F
stop" rating, then the front element glass will be about the same
size. With zoom lenses, if the designers want to produce, say, an
F4 lens that has an effective range of 40mm to 400mm, the size of
the front element will have to be large enough for the 400mm end of
the zoom. If a second F4 lens would be designed to have an
effective range of only 40mm to 100mm, the front element's necessary
diameter would be greatly reduced, to 1/4 the diameter. The size of
the glass used for the front element is in part, determined by the
len's greatest focal length.

This means that you're not making a valid comparision when you
compare the 38-228mm Fuji lens with the fixed length Olympus C170
lens, unless the Olympus lens has a focal length greater than 200mm.
You also stated that both lenses are F2.8. Are you sure that the
Fuji's lens is F2.8 over it's entire range? It's probably F2.8 at
the wide end, but at the telephoto end it might be closer to F4,
maybe even approaching F5.6. Assuming that it's F5.6 at 228mm,
compare the diameter of its front element with a fixed length F5.6
lens that's 200mm or slightly longer. THEN you'll probably see
comparable sizes of the glass used for the front elements.
Anonymous

<news> wrote:

>> This is simple, the olympus is a fixed focal length lens (digital
>> zoom),
> the
>> other is the 35mm equiv of a 38-228mm optical zoom. To get the same
>> Fstop on the short end being a tele, the fuji needs a larger front
>> element. Also
>
> Again, the fuji is a zoom camera, but I am asking regarding the
> situation where the zoom facility is actually not used, this is what
> enables me to make a comparison between apples and apples, rather
> than between apples and oranges. So if it helps at all, consider the
> situation where the zoom does not factor into the picture, then, try
>
>> could easily be the fuji has a larger sensor as well.
>
> The CCD of the fujifilm 2800zoom is 1/2.7" , I don't know, do we
> need to calculate it? 1/2.7=0.3703704" by my calculator
>
> For the Olympus C-170 it is 1/2.5", which gives me 0.4"
>
> 0.4" > 0.37" hence it seems the Olympus is actually larger, in any
> case there is only 0.03" in it, so they are pritty much the same,
> yet the front element of the fuji is twice as big???? and both of
> them have the same f stop number, which is 2.8
>
>
>>
>> --
>>
>> Stacey
--
neil
Anonymous

<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
> 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.
>
>
> 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
--
neil

<news> wrote in message news:428f4c87\$2_4@mk-nntp-2.news.uk.tiscali.com...
>> >> Is it always true that a larger lens will have a lower f number ?
>> >>
>> > lense fill focus the light into a stronger intensity.
>> >
>> >-------------------SNIPPED-----------------------------
>>
>> 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.
>>
>>-----------------SNIPPED----------------
>>
>> 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.
>
>
>
As you have already been told, the f number is dependant on the size of the
lens pupil and the focal length of the lens.

Not all lenses use the same design. Not all lenses are designed to the same
quality. Not all lenses have similar amounts of corrections for the various
lens aberrations. Not all lenses have the same number of elements, (bits of
glass) within them. There are a variety of kinds of optical glass, which
have different refractive indexes.

All these things affect the way that a lens is designed and constructed, and
the size of that front piece of glass is just another factor in that.

You can not get round the fact that 2 or more lenses set to the same f
number will produce the same intensity of light at the focal plane, given
that the source illumination is also the same.

What may seem obvious to you, may well not be scientifically correct.

I would not have considered my Nikon f2.8 - 24mm to be user unfriendly, and
it had a front element about the same size as my Nikon f1.4 - 50mm. And it
certainly did not admit as much light as the 50mm, except when they were
both set to f 2.8 or above.

Roy G
Anonymous

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
>
>
>>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.

---
avast! Antivirus: Outbound message clean.
Virus Database (VPS): 0520-4, 05/20/2005
Tested on: 5/21/2005 12:51:53 PM
avast! - copyright (c) 1988-2005 ALWIL Software.
http://www.avast.com
Anonymous

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.

Ummm, actually, the Fuji would have a larger front element mainly
because of the zoom mechanism. Imagine you have two tubes the same
diameter (same f-stop).

Now one of them, you want to be able to put two or three more tubes
around it, so they can slide inside each other and make a longer tube.

The outside tube would have to be wider if you want the innermost tubes
to remain the same diameter.

Now if you're adding glass to turn those tubes into lenses, you'll need
to use a bigger piece of glass on the outer tube of the zoom setup.
It's not a matter of letting in more light, it's a simlple matter of
physics.

---
avast! Antivirus: Outbound message clean.
Virus Database (VPS): 0520-4, 05/20/2005
Tested on: 5/21/2005 12:58:02 PM
avast! - copyright (c) 1988-2005 ALWIL Software.
http://www.avast.com

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?

The F stop is a ratio of diameter of the lens at the nodal point (you don't
have to know what that means) to focal length of the lens.
The lens is always listed by the largest f stop available - the open
aperture. Thus an f2.8 lens can be any focal length.
You can also make lenses of the same focal length with smaller or larger
open apertures - as an example: in 50mm lenses for 35mm cameras there are
many f1.4 models available, but also f1.7s, f1.8s, and even a few f1.2s -
there have been 50mms with apertures as large as f0.95. Most 50mm macro
lenses have a largest aperture of f2.8, and back in the early days of 35mm

--
http://www.chapelhillnoir.com
home of The Camera-ist's Manifesto
The Improved Links Pages are at
A sample chapter from "Haight-Ashbury" is at
http://www.chapelhillnoir.com/writ/hait/hatitl.html

<news> wrote in message 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 ?
>
> 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
>
>

> > 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.
>
> The problem you're having is due to not comparing two similar
> types of lenses. Your assumption is valid for comparing two fixed
> length lenses. If they have the same focal length and the same "F
> stop" rating, then the front element glass will be about the same
> size. With zoom lenses, if the designers want to produce, say, an
> F4 lens that has an effective range of 40mm to 400mm, the size of
> the front element will have to be large enough for the 400mm end of
> the zoom. If a second F4 lens would be designed to have an
> effective range of only 40mm to 100mm, the front element's necessary
> diameter would be greatly reduced, to 1/4 the diameter. The size of
> the glass used for the front element is in part, determined by the
> len's greatest focal length.

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 ...

Thanking all those who answered (some of which may have learnt something
as well).

>
> This means that you're not making a valid comparision when you
> compare the 38-228mm Fuji lens with the fixed length Olympus C170
> lens, unless the Olympus lens has a focal length greater than 200mm.
> You also stated that both lenses are F2.8. Are you sure that the
> Fuji's lens is F2.8 over it's entire range? It's probably F2.8 at
> the wide end, but at the telephoto end it might be closer to F4,
> maybe even approaching F5.6. Assuming that it's F5.6 at 228mm,
> compare the diameter of its front element with a fixed length F5.6
> lens that's 200mm or slightly longer. THEN you'll probably see
> comparable sizes of the glass used for the front elements.
>

O.k.
Anonymous

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.

<news> wrote:

>> This is simple, the olympus is a fixed focal length lens (digital zoom),
> the
>> other is the 35mm equiv of a 38-228mm optical zoom. To get the same Fstop
>> on the short end being a tele, the fuji needs a larger front element.
>> Also
>
> Again, the fuji is a zoom camera,

And again the olympus is NOT a zoom camera.

> but I am asking regarding the situation
> where the zoom facility is actually not used,
> this is what enables me
> to make a comparison between apples and apples, rather than between
> apples and oranges.

But you are comparing apples and oranges.

> So if it helps at all, consider the situation where
> the zoom does not factor into the picture, then, try to answer what I

I did, you aren't listening.

> so they are pritty much the same, yet the front
> element of the fuji is twice as big????

Again because it is an OPTICAL zoom camera.

--

Stacey
Anonymous

<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?

>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
Anonymous

<news> writes:

>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)

That's generally true of long focal length lenses, where the entrance
pupil and the front lens element are basically in the same place.

But your intuition can be seriously wrong for wide angle lenses. If the
entrance pupil is buried deep in the lens (and it can be for zooms), the
front element may have to be several times the size of the entrance
pupil in order to avoid vignetting.

For an extreme example of this, look at fisheye lenses. They often have
enormous front elements even though the focal length is short and
f/number is modest. The element is large to get *field of view*. Only
a tiny fraction of it actually collects light for any one point on the
image.

Dave
Anonymous

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.