Originally posted by Adam Unless you're always shooting wide-open in Av mode, M lenses require you to stop-down meter (press the green button to set the correct shutter speed) and you can only use them in M (and Av if you're shooting wide-open).
A lenses allow you to use all the camera's shooting modes.
I was a little confused by this... first I see now the camera refuses to jump into any other mode beyond M and Av -- even if you set it to Tv (for example), the camera reports Av mode!
Second.. even in Av mode, I set the lens to f/2.8 take a shot.. 1/15s f/2.8 ISO 800 looks good. I then adjust the aperture to f/22 and take another shot at the same scene in the same lighting. resultant shot still looks good and still reading 1/15s ?!
Then I realized the camera is still using f/2.8 regardless of the aperture selected on the lens. Only until I changed to Manual M mode and pressed the green button did the camera actually read the scene correctly and adjust the setting.
Ok that makes more sense... a little subtle what the camera was doing there.
Thanks, all, for the responses. Learned something new tonight.