EXCERPT page containing first few paragraphs. 2017-04-28 06:55:57
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Wide open, performance is high over most of the frame, but stopping down to ƒ/5.6 pops the contrast to a very high level over the central 3/4 of the frame, with a graceful drop to the edges and corners.
Since MTF ƒ/4 is not supplied, we cannot be sure if most of the gains of ƒ/5.6 are already present, but most likely there are some benefits to ƒ/5.6 over ƒ/4, not the least of which will be concern over focus error, which is almost a certainty with a 135mm lens and rangefinder focusing.
This is a very high performance, but less uniform and less good to the edges and corners as compared with the 90/2 Summicron-M ASPH. The astigmatism that is seen at ƒ/5.6 and particularly at ƒ/8 is apparently due at least in part to lateral chromatic aberration, as per field shots which reveal this color error on higher resolution mirrorless camera bodies.
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Since 2009, Diglloyd Guide to Leica is the authoritative review and reference for Leica M lenses: Leica M, Zeiss ZM, Voigtlander, as well as Leica M digital bodies. Leica S is also included.
It includes hundreds of pages of exclusive coverage and high resolution Retina-grade examples covering Leica M-System lenses and digital bodies with in-depth performance comparisons and analysis.