I haven`t noticed exposure issues more so AF problems. I noticed mine would not AF continuously while depressing the shutter button. I then went to Google and searched for `Tamron 17-50 AF issues`and sure enough it`s a common problem, definitely not something you want to be happening if you rely on this lens for work. The last wedding I did I had to MF the whole day and at times it was a pain.
I`m tempted to contact the company regarding this although I don`t qualify for warranty repair as I bought mine used off a forum member here. I tested the kit lens to be sure it wasn`t the camera and it locked on every time no matter what I was pointing at. I`ve also noticed if using the menus or previewing images with the Tamron mounted the screen may shut off, especially if I shake the camera. This doesn`t happen with any of the other lenses.
Anyway, something to consider or check for if you do buy. Besides that it`s a great lens but I wish I had a DA* 16-50 instead for the fact it`s weather sealed, I have a piece of dust on the inner element of my Tammy now but I`m famous for not putting the end caps on.