Practical experiences with the 16 mm Fisheyes on digital cameras do not support your conclusions. In particular the 16/3.5 images are remarkably sharp.
I agree with this.
Anyone who disagrees should first learn how to spell Rollei correctly
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Looking at samples posted with the CZ lens I do not see anything the old Nikkor could not do in regards to corner sharpness.
According to information found in some Leica forums, the Leitz Fisheye-Elmarit-R 16 mm is the very same lens as the Minolta MC or MD Fisheye Rokkor 16 mm 1:2.8, just in a Leica R barrel. Eric Lund might have read the same comments being active in these same forums. As far as "price, not for us mortals" is concerned, I paid more then $700 for some of the lenses I owned, which seems to be the going rate for the Leica R fish eye.
So, as far as first hand experience goes, Rick, if you do not nescessarily need the 180 degree field of view of the newer lens design, the 16mm f3,5 will be amongst the sharpest lenses you can find for your Df and Nikon F. Period. And at 170 degrees and 0,3m it is plenty wide for what I use it for. It is also a very small lens, and because of that I bring it more often the the UD Nikkor
Picture of the lens for Richard:
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Picture taken with the lens (f5,6):
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f8
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Close focus
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