The setup reveals the color correction differences between the Zeiss 135mm f/2 APO-Sonnar and the Zeiss 100mm f/2 Makro-Planar.
The “APO” designation is not a well defined concept, being used loosely by so many manufacturers and having a different standard for each, so actually looking at the differences is the best way to understand what the upgrade to APO means.
The APO designation is not strictly a finished image quality issue: focusing accuracy is critical on high resolution digital, and so the elimination of longitudinal chromatic aberration (LOCA) is a very practical benefit of APO that the APO-Sonnar offers and that the 100/2 Makro-Planar does not, LOCA being the one significant shortcoming of the 100mm f/2 Makro-Planar.
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Since 2007, Diglloyd Guide to Zeiss is the authoritative reference and review for all of the Zeiss ZF.2 (for Nikon) and Zeiss ZE (for Canon) DSLR lenses, including the Otus line.
It includes hundreds of pages of exclusive coverage and high resolution Retina-grade examples, in-depth analysis of sharpness and contrast, color, bokeh, vignetting, distortion, MTF, and flare, and recommendations and practical usage tips.