Reader Question

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.

Leave a Reply

Your email address will not be published. Required fields are marked *