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EXCERPT page containing first few paragraphs. 2019-05-25 13:29:39

Magenta/green color fringing @ f/2
Nikon D3 + Zeiss ZF 100/2 Makro-Planar

Secondary Longitudinal Chromatic Aberration (Color Bokeh)

The previous page discussed longitudinal (axial) chromatic aberration (LOCA).

Secondary longitudinal chromatic aberration is really just LOCA, but it is distinguished here for its seemingly different effects. A lens which is highly corrected for LOCA will also be highly corrected for secondary color errors (SLOCA).

According to Dr. Nasse of Zeiss:

All these focusing effects are summarized under the name of “secondary spectrum”, so SLOCA is a tautology. In the days of Newton “spectrum” was a general term for an image, and it was in particular used for the colored band of light, visible after passing white light through a prism and separating it into its constituents. This separation of the colors could be compensated by a second prism. However, when it was made from different glass the scientists learned, that the recombination was not perfect. There was still some band of light, the secondary spectrum. Thus this term became the key word for all effects caused by the mismatch of dispersion curves.

SLOCA as discussed here is partly caused by a large amount of LOCA, or by the sphero-chromatic aberration (also called Gauss aberration) of lenses; it does not exist in prisms. In real, fast lenses not only the best focus shows some variation with the wavelength, but in addition the quality of the colored image in its best focus. Very often the blue light has much more spherical aberration. This causes an extended halo at high intensity edges with some designs.

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Intrigued? See Focusing Zeiss DSLR Lenses For Peak Performance, PART ONE: The Challenges, or (one topic of many) field curvature.

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