If you're shooting in AV mode, the camera is using available light to determine the shutter speed and ISO. If you're choosing something like f5.6 or f8 for a group shot, with typical indoor lighting, the camera will first pick the lowest hand-held shutter speed it thinks it can get away with for the lens you're using, which might be as slow as 1/30, and then it will start bumping ISO to get the exposure right, if lowering the shutter speed isn't enough. Also, if this is the "mark I" version of the 24-70, I never found mine to be amazingly sharp (the mark II is better in that regard). I suggest you shoot in M mode, set the aperture to something like f5.6, the shutter speed to 1/125, the ISO to 200 or 400 or even 800, depending on the indoor lighting, and then adjust the flash to get the exposure you want.