UNDERSTANDING F-STOPS... [Archive] - Canon Digital Photography Forums

Steve Parr

5th of May 2005 (Thu), 21:45

This may seem kinda' basic, but I haven't a clue...

My question pertains to something I thought about after reading that the IS feature is "good for an extra f-stop or two".

How much is a single f-stop? Would it be like going from a 3.5 to a 2.5? Half that?

Thanks much...

Steve

Belmondo

5th of May 2005 (Thu), 22:02

Unfortunately, the f-numbers are logarithmic, but they are easy to remember. Each f-stop doubles or halves the amount of light passing through the lens depending on whether you're increasing or decreasing. We all know that the larger number represents a smaller opening, hence f/8 will allow less light than f/4, for example.

Starting at a lens with a maximum aperture of f/2.0 as an example, the next full f/stop smaller would be f 2.8, or roughly 1.4 times the maximum aperture numerically. Two full f/stops would be f 4.

The progression actually goes as follows:

f/1
f/1.4
f2
f2.8
f4
f5.6
f8
f11
f16
f22
f32

Each full f/stop allows 1/2 the light

The significance of the 1.4 number is that it is roughly equal to the square root of 2, thus the non-linear progression of numbers. Every other number is roughly doubled. They fudged a little when they settled on f/11 instead of 11.2, and f/22 instead of f/22.4, but these are really guide numbers more than absolutes anyway.

Most modern digital cameras can break down f/stops into halves and thirds, so you'll see all manner of strange numbers when you're shooting in Tv mode, and even Av mode.