While researching lenses for my new 60D, I found the need to be able to compare the speed of two lenses. I was considering Canon’s three 50mm lenses and wanted to know just how much faster the L series lens was than the others. The mid priced 50mm (MSRP $399) is an f/1.4; an aperture value easy to calculate because it is a full stop slower than f/1 and a full stop faster than f/2. But what about the L series lens’s f/1.2 aperture (MSRP $1600) and the “thrifty 50’s” f/1.8 (MSRP $125). The f-stop scale is not linear, so you can’t just divide them to compare their speed. That is why I made the following chart. It shows full stops, 1/2 stops, 1/3 stops, and 1/4 stops. And since the numbers we see in a camera are approximations, I included precise values to the third decimal. This proves to be useful since, as I found out later, squaring the ratio of two f-stops does give a proper comparison, and failing to use precise values greatly amplifies the error. Bottom line is f/1.2 is 2.246 times as fast as f/1.8 and 1.414 times faster than f/1.4
To use the chart, 1) locate the two f-numbers for comparison, A and B, 2) divide the exact f-number of B by the exact f-number of A, 3) square the result, and you now have how many times faster A is than B.
Also, read this for more information about how to compare f-numbers.