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Samuli Vahonen wrote:
In theory if LoCA is severe enough at focal plane I don't see no reason why this would not show up in focal plane also. In practice of course I have never seen this, and I would assume that applies for majority of people working with camera optics, due to which reason "most authorities" call focal plane CA as "CA" or "lateral CA" and this bokeh thing "longitudinal CA".
Longitudinal CA shows up in the focal plane all the time. It's the classic cause of the "Purple Fringing" effect so often erroneously blamed on sensor bloom. As a general rule, if the colour fringe is the same radially, and diminishes when a lens is stopped down, it's longitudinal CA - a failure to bring enough of the spectrum to common focal depth. Lateral CA, on the other hand, is a failure to bring enough of the spectrum to common magnification. It results in colour fringing that differs radially and generally does not diminish as a lens is stopped down.
A lens with low Longitudinal CA can still show significant "Defocus CA" because although the different parts of the spectrum did eventually arrive at common focus, it's very common for them to take significantly divergent paths through the lens before arriving at that focus. This allows some colours to become concentrated or diluted radially within the light cones, ahead of or behind the focal point. This becomes obvious when high contrast areas are imaged out of focus.
To my knowledge, "Defocus CA" doesn't have an official term because the people who named the various aberrations were all either microscopists or astronomers. As a rule, these two fields only care about the qualities of the focal plane.
A series of spot plots from someone with access to Zemax and a reference design would probably make that much clearer at a glance than I can put it in words..
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