View Full Version : Definition of what the 'f' number means... & how calculated...?

23rd July 2010, 04:04 PM
Hi All

I was asked a question the other evening that I was not able to answer, and the question was; "what exactly does fn.n mean"?

Now of course, I was able to explain what aperture is, it's relationship with exposure, DoF and shutter speed etc, but I have no idea how the actual value for a given lens is actually calculated.

What is the mathematical formulae for calculating the aperture of a given lens? Is is something like the ratio of physical aperture size to distance from sensor or something like that?

Can some tekno person help me here please...?


23rd July 2010, 04:29 PM
Isn't it = f/D where f is the focal length and D is the diameter of the aperture... So a small hole = big "f" number, and 200mm lens with a bay window on the front is still only f4 :mad:

That only works for really simple lenses mind you. Quite how they calculate it with the ZD8mm who knows ...


23rd July 2010, 04:32 PM
Yes, just found out from Ian that the f number is basically the focal length divided by the aperture diameter.

23rd July 2010, 04:40 PM
And before anyone asks - I believe the f stands for focal ratio.

23rd July 2010, 06:53 PM
Have a look at