Ever wondered what "longitudinal chromatic aberations" (or short: longitudinal CA) is and how it looks like?
Now, here is a typical example:
You can easily see, what the problem is: in front of the point of sharpest focus things get a violet tint, behind a greenish coloration. This is also sometimes called "Bokeh CA" as the out-of-focus areas are affected. This sort of CA is very hard to correct, as no programm can identify whether a given unsharp pixel-cluster is in front or behind focus.
This is in stark contrast to lateral CA which produces a colored fringe of a certain width (typically1-2 pixels) to one side of the original high-contrast border and can be quite easily corrected in-camera or with post-processing.
Not always is this effect distracting, as you can see in the following shot. The bokeh CA can be seen as a nice green "halo" around the head of the dandelion.
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