Originally posted by emalvick But each focal length will have a different curvature. How do you remedy that?
forget field curvature for a minute, the reality is lens designers consider this a good trade off for lens sharpness since its not all that visible if your not looking for it in the first place and in fact zoom lenses often trade one or more aberrations for higher image sharpness.
light also disperses, even a focused beam will lose intensity as the beam will get wider over distance, you might not think this is a big deal but that is only one of many factors that is causing peripheral fall off and drop in contrast at the edges of the frame
the one i believe that causes the most problems is angle of light and is also why i think your argument for curved sensors only benefiting fixed focal length cameras is a bit closed minded.
take a torchlight shine it on a flat piece of paper from 4 inches away at a dead on angle you just a nice round patch illuminating the paper evenly, the moment you angle the light the patch is no longer round now nor evenly illuminated, repeat the same test from different distances 2 inches away and 6 inches away (this is like zooming in and out) the results will be the same (this test is actually how you demonstrate the effect of a aberration known as coma)
now repeat the test with a empty fruit bowl and right away you will see the benifit the moment you change the angle, of course one distance will work better than the other but they all are receiving the benefit of illuminating a curved surface (the size/shape of the patch is better controlled and there is less fall off from one edge of the patch to the other vs the paper test.
now back to field curvature, yes zoom lens has varying curvature but its far away from being flat, but the fruit bowl results should show you that a curve surface can in fact benefit zooms, it might be only a little but any thing that helps flatten the curvature has a list of other beneficial effects on image quality (coma resistance, lower peripheral fall off (vignetting), reduce edge astigmatism) those very benefits would allow for future lens to require far less elements, most existing zoom lenses do not correct for the benefits a curved sensor could offer anyway, the only incompatible lenses would be high quality primes :P