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Mike K wrote:
Here is a table comparing MTFs from Zeiss lenses; I presume the tests were performed on an adapter modified Canon 5D for FF coverage?
The 25/2.8 performed well at the center, very close to the 21/2.8 and even better at the smallest apertures. However at the border (zone B) the 21/2.8 had consistently higher MTF values over the 25/2.8. Without more details it would appear to be an apples to apples test.
OK. Here is the obvious question. At what distance were those MTF's produced? As has been discussed here with respect to another self-appointed web tester (photozone), the ZF 25mm does not do well reproducing test charts at a distance of a couple feet. It's not designed for that. It is designed, primarily, as a landscape lens and accordingly, is optimized for infinity.
In addition, the source of these tests suggest that these are tests of the Contax c/y Zeiss lenses NOT the ZF/ZE lenses!!! the test of the 28/2.8 should have been a dead give away (ZF version doesn't exist), the same goes for the 18/2.8(!).
However to get back to the discussion, at infinity or typical landscape distances, Herb's findings are entirely possible, and not inconsistent with my experience. Looking at Zeiss MTF's, which are done at infinity, at f5.6 the ZF 21mm and the ZF 25mm are very comparable. The 25mm could indeed perform better the the edges of the frame at f5.6 (might depend on the nature of the subject, to some extent), while we might expect the 21mm to be a little better in the center.