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Bob Atkins
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Interesting but not surprising. Even a plane filter, between the lens and the film/sensor, acts as an optical element. In front of the lens it doesn't, but in the lens or behind the lens it does. Even an air space is an optical element.
Lenses designed for film assume nothing between the lens and film, so there will be some modification of optical performance, and the shorter the focal length and faster the lens, the larger the effect will be.
But there's nothing you can do about it. If you want to adapt an older, manual focus lens designed for film cameras, you get what you get. You can't change the optics of the lens and you can't change the optics of the sensor (unless you want to use a bare sensor with no AA filter or protective cover - and that would be a nightmare to keep clean). In theory I guess you could add some compensation optics, but it simply wouldn't be worth it. Might as well buy a lens designed for the camera!
Just one more thing for the sharpness obsessed to worry about!
I wonder if lens designers like Zeiss who are making MF lenses for EOS and Nikon have different optical designs taking into account the filter thickness. I presume not. They make one lens with one optical design.
Does it matter? Well, looking at the numbers it seems that different Nikon and Canon cameras have different filter thicknesses. Nikon and Canon just make one version of each lens which will fit any of their camera bodies. They don't sell one version for the Nikon D1x and a different version for the D200, even though the quoted numbers show quite different physical filter thicknesses (0.7mm vs. 2.2mm).
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