I need some help with my camera. It's a Rebel XT. Today I turned it on, put it on Av and the aperture was not changing. Then I tried changing it using the battery grip and it worked, and its working again on the main dial. But now I feel like its giving me wrong settings. I have the aperture a f/2.8 and iso at 100 and its giving me shutter speeds of 1/25-1/30 in a dim room. The pictures are coming out dark. I dont know why its doing this, it should be giving me a slow shutter speed, but its not. Any suggestions?