Paul Franklin Stregevsky said: > Chris Brogden <[EMAIL PROTECTED]> wrote: > > "I believe it's only a half-stop from 1.2 to 1.4." > > Here's how f/stops compare. I don't remember where I got these numbers. I > may have derived them, so feel free to question them. > > f/1.2 is 0.45 stop faster than f/1.4 > f/1.4 is 0.62 stop faster than f/1.8. > f/1.4 is 1.0 stop faster than f/2.0 > f/2.0 is 0.67 stop faster than f/2.5. > f/2.5 is 0.33 stop faster than f/2.8. > f/2.8 is 0.7 stop faster than f/3.5. > f/3.5 is 0.3 stop faster than f/4. > f/4 is 0.25 stop faster than f/4.5. > f/4.5 is 0.67 stop faster than f/5.6.
f/stop is the ratio of focus length to aperture diameter, and it all follows from there. The light hitting the film is inversely proportional to the square of f/stop (the aperture area), so to double the exposure you'd go, for instance, from f/1 to f/1.4, since (1.4/1)^2=2. And (2/1.4)^2=2, (2.8/2)^2=2, etc. That's why they chose those funny numbers to mark on the barrel. And if you can Taylor-expand the square root you can show that half way between two stops is close enough to half a stop. So going half a stop up from f/2 is technically f/2.45, but halfway between the numbers is f/2.40, which changes your exposure by 4%, which is not enough difference to matter when your exposure compensation goes by 33% or 50%. Going up by halves, f/1.0 f/1.2 f/1.4 f/1.7 f/2.0 f/2.4 f/2.8 f/3.4 f/4.0 f/4.8 f/5.6 etc. I may not be a great photographer, but I know physics.
