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Is there a huge difference in quality say between a...
Carl Zeiss and a Schneider-Kreuznach compared to a DSLR lens on a Nikon or Canon?
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There are user reviews at
http://www.photodo.com/products.html that have very high ratings for Carl Zeiss. But that site has no standardized tests for Carl Zeiss.
However, that site reviews across lens formats and the highest rating they've ever given is to a Canon EF lens. In general, the best Canon and Nikon lenses fare well against even the best Leica lenses. But there are no bad or even Leica lenses, and there are average Nikon and Canon lenses
Schneider, isn't that a Large format lens? Why would one compare a large format lens to an SLR lens?