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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