This following question has bugged me for years, and lately I have been thinking about it again as I ponder the possible purchase of a 70-200mm 2.8. I know that the calculated physical size of the aperture of a 'fast lens' at f8 is the same as that of a 'slow lens' at f8 for a given focal length, and thus the calculated proper exposure SHOULD result in the exact same exposure time. However, with all of the modern lens technology, all the various glass inside, all of the various coatings, etc, is the calculated exposure time really identical for both lenses at the same f-stop? I just don't see how this could be true given the very different construction. Yes, I could test this theory but I guess I am not *that* bugged by the question and am just seeking the wisdom of the board. In essence, I want to defy physics and insist that 'fast' lenses have faster shutter speeds at the same f-stop and focal length than cheaper or 'slow' lenses. Then of course, the follow-up question. Why don't manufacturers make high optical quality lenses that do not have those fast apertures? Smaller, lighter, less expensive, but the same or even better quality at f4 and above.