Originally posted by SpecialK Knowing what the meter is thinking - and correcting for it if needed - is far more important that the actual mode used, such as M, Tv or Av.
This is very well expressed:
A camera can't read your mind and know which parts of a scene you want to optimally expose for. For all the camera knows, the sky is your main subject, and everything else is meaningless.
You'll learn to look at the range of brightnesses and grey levels in a scene and know when you'll have to use some form of compensation.
A girl's torso against a grey wall, no. A girl's torso with the bright sun behind her, yes. Same subject, but the other light elements entering the lens and affecting your metering are vastly different.