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EXCERPT page containing first few paragraphs. 2022-08-11 13:22:35

Axial chromatic aberration
Nikon 180mm f/2.8D IF-IED @ f/2.8

Longitudinal Chromatic Aberration (LOCA / Violet Fringing)

Longitudinal (axial) chromatic aberration (LOCA) is a type of color error whereby the focusing of light varies in focal distance by wavelength: the red, green and blue areas areas of the spectrum focus at slightly differently distances. Thus if the lens is focused optimally for red or green light, blue light might be slightly out of focus, leading to a halo effect. And vice versa. Typically the design compromise is in the blue/violet area of the spectrum, hence violet halos/haze sometimes referred to as “purple fringing”.

The result of differential color focus (focal distance), LOCA presents as an overall veiling haze at full aperture. It also presents as a high contrast violet halo, as shown at right. In general, it manifests itself more subtly as a reduction in overall contrast, and a drop in brilliance; the look is very similar to spherical aberration. For an otherwise well-corrected lens, LOCA is often the limiting aberration at full aperture. The astute reader might note that spherical aberration by color (spherochromaticism) is a natural combination of both, often with unpleasant visual result.

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

Actual image of light from a prism.
Lateral chromatic aberration is esssentially a prism-like effect;
longitudinal chromatic aberration occurs for similar reasons

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