...hmm...scratches body part... If I understood the question, Don gave the right answer, at least for focal plane shutter SLR's. The lens is already stopped down before the little slits go winging their way across the film plane. Now, for in the lens shutters, hmm... Again, the aperature is steady before the shutter opens. But the shutter blades are like sphincters, [hows your gas, BTW?]. I think bsteph must be close. There 'prolly is a small amount of different exposure across the whole frame, but not enough to make any difference compared to the total time the shutter is open. Can't follow the temperature argument myself, but I think exposure does change a small amount with temperature.