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Violet Fringing (LOCA) vs Aperture (Sunlit Water Bottles)
Readers sometimes inquire about the violet fringing seen with the 100mm f/2 Makro Planar. This color fringing is longitudinal (axial) chromatic aberration, sometimes referred to as LOCA (see Making Sharp Images for more on color aberrations).
LOCA
LOCA is essentially a differential front/back focus for different wavelengths (blue/violet focuses just a bit differently than green and red, thus forming slight haloes around fine points). LOCA responds quickly to stopping down, with depth of field overcoming the focal differences between the blue/green/red spectrum. LOCA is always present even fast (bright) aperture lenses; even the most expensive Leica lenses with their exotic and expensive optical glass types (any of the M f/1.4 ASPH wides, the Noctilux-M ASPH f/0.95, etc) and even Leica S lenses (the 70/2.5). LOCA is simply one of those things that is unavoidable once a “fast” lens design is chosen. The Nikon 180/2.8 ED-IF has quite strong LOCA, and it is an f/2.8!
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