What is the difference between F- stop and Aperture Value


Horst

Link Posted 27/01/2013 - 10:57
When looking at the galley pictures and checking the Exif data, I find the F-stop and the aperture value to be different.

Why is this so?

Regards, Horst

Bracken

Link Posted 29/01/2013 - 17:54
I too am puzzled. Surely someone on here knows the answer.

JohnX

Link Posted 29/01/2013 - 18:26
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

Bracken

Link Posted 29/01/2013 - 18:50
Many thanks. Now I understand - well, almost.

Horst

Link Posted 03/02/2013 - 12:40
Quote:
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.


Thanks for that. it took me a while, but I think I understand it now.

Regards, Horst
Add a Comment
You must be registered or logged-in to comment.