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Diglloyd (Lloyd Chambers) have done a test and review of the Zeiss 135 lens. More than 25 pages and many pics also. He also compare it with the Canon 135 lens. You need to subscribe to read everything of it. Here is a little bit from the conclusion.
The 22-megapixel Canon 5D Mark III is used here to compare the Zeiss 135mm f/2 APO-Sonnar to the Canon EF 135mm f/2L. The Canon lens has long enjoyed a well deserved reputation as a first class performer, so how does it stack up against the Zeiss APO-Sonnar?
The Zeiss 135mm f/2 APO-Sonnar shows its strengths: it offers a very high level of micro contrast even wide open with no longitudinal chromatic aberration, minimized secondary longitudinal chromatic aberration and outstanding correction of off-center aberrations that results in near-circular specular highlights even wide open. Truly an impressive performance that few lenses can equal.
The Canon 135mm f/2L is surely a fine lens, but it shows its weakness here at wider apertures (uncorrected color and point aberrations), weaknesses that would be even more apparent with a demanding lighting situation and/or on an as-yet unavailable ~40-megapixel Canon DSLR. However, stopped down to 5.6, the Canon 135/2L is a fine performer, and the difference diminish greatly (most aberrations diminish greatly by /5.6).
One factor remains however: photographers stopping down to /4 - /8 and finding autofocus useful or essential might still favor the Canon lens for its autofocus capability."