SJMD,
Good point about AV mode, and holding the position constant with a tripod. Otherwise AV mode + exposure compensation can throw you for a loop.
What burned me in the early days was using AV mode, gettings an incorrect exposure, diaing in flash(FEC) or exposure comp(EC) and not getting the expected result. This is because the EFC, or EC, is relative to the camera's current exposure calculation. So if you move a bit, and the exposure measuring area changes, you are dialing in EC to a quite different exposure.
This is why I often tell people to see what P mode would chose, then use those settings in M mode, and vary to taste. That way, you can vary your exposure with fstop, and dialing in FEC has predicatable results.
