F stop / Aperture value?

In the gallery when you look at the Exif it shows the F stop and then the Aperture Value.They are usually different. Sometimes bigger and sometimes smaller. can anyone explain this please.
CHEERS Vic.

I'd never noticed this Vic, don't tend to look at exif that closely.

However, now that you've pointed it out - that's interesting!

I can see a couple of different effects - presumably down to the software used to generate the image.

Looking up the details for EXIF I found :
F-stop should be the aperture used to take the picture.
Aperture Value is a calculated value which is defined as 2x ln(f-stop)/ln(2)
(maths here: http://en.wikipedia.org/wiki/APEX_system )
f/4 will work out the same for both though.

It does look as if some software incorrectly puts either the max aperture or F-stop value into the Aperture Value field though. Bear in mind that depending on the mode of shooting exposure compensation might result in slightly funny values as well.

The only time f-stop and aperture value are the same is at infinity. f-stop is the value of the focal length divided by the opening, but when you change the focus, the actual focal length changes somewhat, so the indicated opening and the aperture value are different.

Whilst true for many lenses, not all lenses change focal length. Some of mine, especially older primes, simply increase lens-sensor distance and have no way of altering the relative spacing of elements without taking them apart.

ALL lenses change focal length a little bit while focusing. The increase in the lens extension while focusing changes the focal length. And the extension of the lenses while it focuses closer reduces the amount of light that reaches the sensor.

I thought the F stop was exactly as set in AP OR exactly what the camera chose if if P. OR if in P or User then when you move the wheel the aperture is set and that is what the exif reads as F stop. As per [blob(name?)]
SO when you focus, the F stop remains physically fixed but the amount of light passing through it is actually either more or less than the fixed F stop value.
SO if this is the case then if you focus after setting the F stop then you will almost never get the correct exposure as the amount of light passing through the aperture is not correct.
Have I understood this correctly or am I way off?
Thanks to blob, Yves, and Mike for trying to help.
CHEERS Vic.

You are about right. At infinity, the marking and the metering value are accurate. But when you focus closer, there is less light reaching the sensor than at infinity. BUT the change is too small to make a pronounced difference on the actual picture, unless you use a macro lens and take macro pictures. Then, the extension of the lens makes a rather large difference, but some macro lenses have a diaphragm mechanism that will "slide" the opening to a larger physical opening to compensate the extension.

Well I never. Here's me been taking photos for all these years thinking that the aperture that was set was the aperture being used. SILLY ME.
It's all as clear as mud to me. Thanks for trying to educate a poor simpleton like me.
CHEERS Vic.

I wondered about that too Vic, so thanks for asking. And thanks for the answers gents

There's another current thread on the exact same topic in which I posted this answer. Hope it helps?



Take as an example an F-stop of f13. The corresponding "aperture value" will read 7.4.

Why? Av is formally defined as 2 x log(base2) of the aperture. So 2 x ln(13)/ln(2) is ~7.40.

Why log(base2)? Because each whole number increase represents a doubling, i.e. one stop.

Why multiply by 2? Because the amount of light let in by a given f-stop is proportional to the aperture's area rather than its diameter, so the aperture squared is the key variable.

A fuller explanation is here:

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

I've got a headache now

I'm sorry I asked! JohnX. You've lost me now. But thanks for trying.
Stan have you any spare Pracetemol?
CHEERS Vic.

Sorry guys! Have a Happy New year instead