Originally posted by k100d was looking at FA Limiteds and was wondering how many stops difference they were from other standards apertures. So looked around and based upon the formula on Wikipedia for Aperture Area, the formula is as follows:

Difference in Stops = Slower Aperture^2 / Faster Aperture^2 - 1

my DA21 e.g. is f3.2, so 3.2^2 / 2.8^2 - 1 = 0.31, so about 0.3 stops slower than f2.8

thought this might be handy...

So.... another way to do this is to find out the ordinal (counting) value of the stop. By that I mean, count with f/1 as the 0th stop, f/1.4 as the first, f/2 as the second, etc. This can be done with this formula:

**log(***stop number*)/log(sqrt(2))

Which gives, rounded to two digits, this:

*Code:*

f/1.4 = 0.97

f/2 = 2

f/2.8 = 2.97

f/4 = 4

f/5.6 = 4.97

f/8 = 6

f/11 = 6.92

f/16 = 8

Which is probably overly precise, because manufacturer's tolerances and just plain fudging are probably a lot bigger than this rounding error. So, f/2.8 is about the third stop. The formula log(

*3.2*)/log(sqrt(2)) gives ~3.36, or, again, about a third of a stop slower.

Maybe this is too geeky, but I find it a more intuitive way to look at it than the simpler but more magical ^2/^2-1 approach. Your mileage may vary.