A reader asks:
A lens – say a Canon L lens that are f/2.8 – is the amount of light the same when it is wide open on an an APS-C sensor vs Full frame sensor? Does it get stopped down on APS-C ?
Great question. No – the light is the same. Area is smaller, but an f/2.8 lens lets in the same light on a smaller sensor that it does on a large sensor. f/2.8 is f/2.8!
Follow up:
Then how does the 2x or 1.4x extender get stopped down when added to 70-200 for example? is it because the barrel gets longer?
Exactly. f/2.8 means the lens’s focal length divided by that is the circle’s diameter, so that does make a difference.
