I think I'm right on this but if you have a 400MM lens and you take a picture with it and then you crop it as if you had taken it with a 800MM lens then that is 100% crop. As I stated I think this is correct, if this is not correct please feel free to explain.golferboy426 said:what does 100 % crop mean. What frame size are you shooting. I've got a Canon EOS Rebel t3i but I shoot smaller frame so I can email