Originally posted by Andi Lo I still find this confusing... so what am I really missing when the ae-l in M mode works exactly like the green button?
Nothing, given that AE-L serves no other function in "M" mode. In "M" mode, it's just a different button to achieve exactly the same purpose. It's only in other situations where the Green button provides anything extra - like returning to the default setting quickly after fiddling with some parameter.
Quote: I'm assuming that when you use Av or Tv the green button will set the aperture and shutter speed for you. (like program mode)
When you use Av mode, the green button is not needed - the dial controls aperture (Av) or shutter speed (Tv) and the camera sets the other parameter automaically. But if you've dialed in exposure compensation, hitting the Green button resets it (actually, I think it has to be Green button + exposure compensation button together). Just a little shortcut.
Bottom line: you aren't missing much.