> The question is- are AF > lenses designed with compromises in order to achieve high focusing > speeds? I am not talking about using polycarbonate, or feeling loose > on MF but in the amount and types of glass used. The two most obvious things I can think of are internal focus (IF) and shortened focus throw. IF is not new with AF and offers other benefits, like smaller lenses, closer minimum focus, minimised speed loss at close focus and better hand holdability. The shortened focus throw has no direct relation with optics, although, being combined with loose focusing ring, it makes MF much more difficult. > What I wonder is- if Nikon designed the lens as a no compromise > optical design, would it be the same, or would heavier elements with > different refractive properties be used? We seem obsessed with > focusing speed these days and I just wonder if this means we are > accepting optical compromises-i.e. given a free hand, could the > designers produce sharper, contrastier lenses? Lens design is about compromises. Usually, the more the lens is complex, the more you need elements to correct aberrations. And more elements means a more complex lens... If you forget about zooms, best lenses would be near-symetrical lenses like the ones used for large format cameras. Add ED/APO glass and you get tack sharp, virtually distortion-free lenses, and simple designs (it is always recommendable to keep the number elements to a minimum). Drawbacks: important light fall-off, modest speed, and lens must be about the focal length from the film (problem with long and short lenses, incompatible with SLR designs). The supposedly sharpest 300mm Nikkor ever was the 300mm f4.5 ED non-IF. Tack sharp from full aperture, even in the corners, but the beast weighed a ton, only focused down to 5m and was slow to focus. Newer, "compromised" lenses like the AF ones may be slightly softer, but they are much more usable. While I would not care too much about compromises made to speed up AF of the new 80-200 AFS (wich I can't afford anyway), I would mind more about the ones made in a 28-200 (or even worse, 28-300) lens. Hope this helps, Nicolas.
