hagerman@nermal.DEC (06/15/84)
x re: lenses sharpest at medium f/ stops. If you don't think it's possible to build a lens that is sharpest at its widest opening, consider this: My lens is sharpest at, say, f/8. I put a mechanical stop on the aperture (note spelling) control so that it can only move between f/8 and, say, f/16. Now I have a lens that is sharpest at its widest opening. If this sounds far-fetched, try measuring the diameter of the front element of some of your lenses and then calculating what the maximum f/ stop should be. You'll find, in many cases, that the front element is larger that you would expect. (Specific example: Leitz Summitar 50mm f/2.0 works out to about f/1.5.) This is to minimize vignetting problems. The manufacturer can choose between making a lens that is faster but not quite as sharp or a slower lens that is sharper. In fact, Leitz claims that their 50mm f/1.0 is as good as their 50mm f/2.0 lens. The problem is that the f/1.0 lens is a monster. The point is that lens design, like a lot of other things, involves compromises, and the result is that most lenses are sharpest at medium f/ stops. D. Hagerman