All SLR cameras are set to the biggest aperture (for example f/2.8 depending on what your lens would allow you), when you set it to a smaller aperture (f/8, f/16, etc) you will still see on your viewfinder at the biggest aperture. Some cameras come with a DOF (Depth of Field Preview) button where you could see (on a trained eye) what your picture would look like if you set it to a smaller aperture. I hope I am making sense, at least that's how it was explained to me. I had a DOF preview on my old Nikon FE but I really didn't use it.