Originally posted by Ash Uh oh...
Sounds like a recurring theme.
This lens is unfortunately notorious for having a sticky aperture lever.
Have a look through the lens whilst it's in A mode on the aperture ring - it should leave a tiny opening (basically an aperture of f/22). If it's gaping open, then the lever's stuck at the wide open position, which it shouldn't be.
If that's the case, and you bought it new, I'd take it back for either an exchange or repair under warranty. It's such a great lens otherwise, though - I'm sore I had to give mine up for the same reason...
Ufortunately Ash is right here annd OP confirms by saying he's getting lot of overexposed pics.
The appreture is stuck wide open and that's the reason. My copy did the same after month or so andd I know I wasn't only one it happened to. It's sad because otherwise it's a great lens.