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Whither Longitudinal Chromatic Aberration?
Longitudinal chromatic aberration (LOCA) presents two challenges: accurate focus, image micro contrast and the dreaded violet fringing (“purple fringing”). Focusing is more difficult with LOCA (ambiguous actually), and the resulting drop in micro contrast is undesirable. Furthermore, violet fringing can be distracting in the resulting image, and might even require correction in some cases (“defringe” or similar). While LOCA improves quickly with stopping down, this is of no benefit if one needs to focus or expose at the brightest two apertures.
The 55/1.4 APO-Distagon is blissfully free of LOCA.
By comparison, the generally well corrected 100mm f/2 Makro Planar suffers from LOCA that results in visible violet fringing and challenging focusing on a high resolution digital camera. In the field, my repeated finding is that the apparent best focus for the 100/2 Makro-Planar in fact is just slightly sub-optimal, a minor focus tweak that looks slightly less sharp in Live View is in fact a better focus for the final image. Hence LOCA can be a serious problem for high resolution imagery (getting the focus spot-on).
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