I was messing around with f stop changes just to see what the A (or Av) mode could really do to the same scene. The camera/lens was on a tripod and the live-view setting was turned on. The ISO was constant at 320 for both shots as was the focal length–400mm as was the exposure compensation setting (+/-) at 0. Changing the f stop affected the shutter speed. Remember this when you’re in a low light setting. Open the lens to increase the shutter speed so you can hand-hold the camera without blur. Check out these pictures:
F8 at 1/40th
F4 at 1/160