Just been reading someone singing the praises of Canon cameras Custom function #4 ( shutter/AE lock button).
Essentially it gives the photographer a chance to use AI servo while also being able to focus and recompose, the best of both worlds sort of.
It also can be used to help affectively separate focusing and metering by allowing you to perhaps spot meter/centre weight meter a subject and recompose while retaining AI servo. The AE lock button is used to switch between the two.
Does anyone use this?
Cheers, Paul 8-)