Lense diamter?

I have read about how the apperture, lens quality, sensor size / type and
all kinds of stuff affects the image in various ways. But I haven't found
any information about how the diameter of the lens affects the images. This
is not used in the specs of the cameras either, is this not an issue or
what? The gut feeling is ofcourse that a wide lense is good and lets in more
light. But even if that is true what does it practically mean? Longer
shorter times? Aperture? Depth of field?

Any information or pointers would be nice.

Tim

Hey I have a question for you... the natural progression of F stops goes down as
8, 5.6, 4, 2.8... but my lens only goes to 3.5 so the numbers on it go 8, 5.6,
3.5

Does this mean that the stops are not exact? I think it does... that means going
from f5.6 to f3.5 is not the same with going from 1/500 sec to 1/1000 sec
exposure... there will be an error... unless they invent their own numbers.

Do you think the actual lens numbers are wrong and the f stops on the lens are
equal spaced, or are the numbers accurate and the last stop wrong?



On Wed, 30 Jun 2004 03:45:22 +0200, Gisle Hannemyr wrote:

"Tim" writes:

I have read about how the apperture, lens quality, sensor size /
type and all kinds of stuff affects the image in various ways. But I
haven't found any information about how the diameter of the lens
affects the images. This is not used in the specs of the cameras
either, is this not an issue or what?

It is, sort off. The diameter of the lens barrel doesn't matter, but
the size of the pupil does. This measure appear in spec. sheets as
aperture, expressed as an f-number (e.g. f/2).

Note, however, that it the absolute diameter in millimeters isn't a
very useful spec. - for reasons that shall be explained below, the
f-number is the preferred measure. But if you want to, you can
compute it. Just divide the focal length with the f-number.
A 50 mm f/2.0 lens has a pupil that is 50/2.0 = 25 mm, and
a 50 mm f/1.4 lens has a pupil that is 50/1.4 = 36 mm,
and so on.

The maximum size of a lens' pupil (maximum aperture) is usually given
as an f-number. The clever thing about f-numbers is that they express
the diameter of a lens' aperture in terms of a fraction of the focal
length of the lens. For example, f/4 represents an aperture diameter
that is one-fourth of the focal length. This way of indicating
aperture is convenient because the f-number is a measure of the light
density that will reach the sensor, irrespective of the focal length
of the lens. In other words - the same f-number will always deliver
the same density of light, so at a given f-number you can set the
exposure time by measuring the ambient light of the scene without
having to enter the focal length into the calculation.

The gut feeling is of course that a wide lense is good and lets in
more light.

Yes - and that's why you have to pay a lot more for a lens with f/2 as
the maximum aperture, than one with f/5.6 - all other things being
equal (smaller aperture numbers means a wider aperture).

But even if that is true what does it practically mean?
Longer shorter times?

Greater aperture (smaller f-number) menas shorter exposure times.
When you reduce the aperture number with the square root of two
(this is usually called "opening up one stop") - you should half the
exposure time. In the old days, before cameras became automatic -
photographers performed these calculations all the time. With auto
modes the camera performs the computations for you - but it still
helps to know what goes on.

Also note that the apertures on most lenses are expressed as a
geometric progression with the square root of two as multiplier
(rounded): f/2, f/2.8, f/4, f/5.6, f/8, etc.

Aperture?

Aperture is a direct measure of of "wide" the pupil in the lens is.

Depth of field?

Opening up the lens pupil narrows the DOF.

To read more about DOF and aperture and other things, visit:
http://dfleming.ameranet.com/

It has an online DOF-calculator where you can see how aperture,
focal length and sensor size affcts DOF:

http://dfleming.ameranet.com/dofjs.html

One thing to note is that compact digicams with tiny sensors need to
use lenses with much shorter focal length than SLRs have - so they get
away with using lenses with much smaller pupils than those needed on a
full frame camera for the same f-number. Because the DOF is a mainly
a function of absolute pupil size - compact digicams have a much
greater DOF for the same angle of view and aperature than a camera
with a full frame sized sensor.

Wider lenses means larger lens elements are used. Larger lens elements
generally equate to a faster lens (Larger maximum aperture).

"Gisle Hannemyr" wrote in message
...
"Tim" writes:

I have read about how the apperture, lens quality, sensor size /
type and all kinds of stuff affects the image in various ways. But I
haven't found any information about how the diameter of the lens
affects the images. This is not used in the specs of the cameras
either, is this not an issue or what?

It is, sort off. The diameter of the lens barrel doesn't matter, but
the size of the pupil does. This measure appear in spec. sheets as
aperture, expressed as an f-number (e.g. f/2).

That was a lot of information, thank you very much for that,
I am just very suprised at the gist of it, that the barel size doesn't
matter.

Tim

"Jimmy G" wrote i
Wider lenses means larger lens elements are used. Larger lens elements
generally equate to a faster lens (Larger maximum aperture).

Okey, so the lense needs to be bigger to get a larger apperture but there is
no need to bother about looking at the barrel size as a general factor?
Given two lenses with the same maximum aperture there is generally no need
to look for a bigger lens, of course it is likley to imply better lense and
bigger senser and what not but given these factors are the same I mean.

Tim

Tim wrote:
I have read about how the apperture, lens quality, sensor size / type
and all kinds of stuff affects the image in various ways. But I
haven't found any information about how the diameter of the lens
affects the images.

That is because it does not effect the image in any direct way. First
you may be talking about several things when you talk about the diameter of
a lens. Do you mean the size of the front element, the mechanical body of
the lens etc.? In any case both of those any many other things (other than
aperture) have no collation with image quality. Larger or smaller does not
mean better or worse. It just means different designs.

This is not used in the specs of the cameras
either, is this not an issue or what? The gut feeling is ofcourse
that a wide lense is good and lets in more light. But even if that is
true what does it practically mean? Longer shorter times? Aperture?
Depth of field?

Any information or pointers would be nice.

Tim

--
Joseph E. Meehan

26 + 6 = 1 It's Irish Math

The effect of lens diameter is tied in pretty much with the relative
aperture (f/#). A large diameter lens on a long focal length
(telephoto) lens has the same effect as a smaller diameter on a shorter
focal length.

In general, the lower the aperture (larger the diameter) at a given
focal length, the harder it is to combat geometric aberrations. It is
typically impossible to correct ALL of them, so all lens designs are
compromises. On the other hand, large diameter lets in more light, for
faster exposures.

As you reduce aperture diameter by stopping down (going to higher f/#),
image quality improves, until you get to very small diameters where blur
from diffraction starts to dominate over blur from geometric
aberrations. Most decent books on photography do go into this.

Tim wrote:

I have read about how the apperture, lens quality, sensor size / type and
all kinds of stuff affects the image in various ways. But I haven't found
any information about how the diameter of the lens affects the images. This
is not used in the specs of the cameras either, is this not an issue or
what? The gut feeling is ofcourse that a wide lense is good and lets in more
light. But even if that is true what does it practically mean? Longer
shorter times? Aperture? Depth of field?

Any information or pointers would be nice.

Tim

--
Don Stauffer in Minnesota

webpage- http://www.usfamily.net/web/stauffer

but there is no need to bother about looking at the barrel size as a
general factor?

Try explaining that to my wife!

On Wed, 30 Jun 2004 12:14:09 +0200, Gisle Hannemyr
wrote:

Bob writes:

Hey I have a question for you... the natural progression of F stops
goes down as 8, 5.6, 4, 2.8... but my lens only goes to 3.5 so the
numbers on it go 8, 5.6, 3.5

Does this mean that the stops are not exact? I think it does... that
means going from f5.6 to f3.5 is not the same with going from 1/500
sec to 1/1000 sec exposure...

First - unless I've misunderstood what you say here, I think you've
got this backwards. Opening up is equivalent to increasing - not
decreasing - the shutter time in terms of exposure. So, for reasons
that will be explained below - opening up from f/5.6 to f/3.5 will
have the same effect on exposure as changing the shutter time from
1/500 sec. to 1/167 sec.

Oh I just meant "1/2" the exposure, not important...

there will be an error... unless they invent their own numbers.

Aperture f/3.5 is just a tad less than one and a half stop below
f/5.6 (the halfway point is actully f/3.36 - but it's close enough
for jazz).

Do you think the actual lens numbers are wrong and the f stops on
the lens are equal spaced, or are the numbers accurate and the last
stop wrong?

The last stop is accurate, and nothing is wrong.

One of the best Nikkors I've ever owned was a 85mm f/1.8 - that's a
about a quarter stop below f/2. It is quite common for the widest
aperture to be an "odd" value that doesn't fit into the normal
aperture progression. You want to have that extra half a stop of lens
speed - don't you? Well, the manufacturer saw fit to provide it - so
they make sure they tell you by marking it on the lens.

If you're on automatic, don't even think bout it - the camera will
figure out the correct exposure.

If you're shooting fully manual, instead of dividing the exposure time
by 2 to keep exposure contant when you open up from f/5.6 to f/3.5,
divide it by 3, and it will be right.

In other words: If f/5.6 and 1/1000 sec. gives the correct exposure,
so will f/3.5 and 1/3000 sec.

OK thanks - you answered the question, it's up to me to know that the last stop
isn't 1/2 the light... or twice the light I should say!!