Chromatic Aberrationtechnical
Color fringing — usually purple/green edges — where a lens fails to focus all colors at the same point.
High-contrast edges (a branch against bright sky) can show thin colored outlines because the lens bends different wavelengths slightly differently. Better lenses control it; it's also a one-click fix in most RAW editors. Stopping down a little can reduce it.
Use this when
- High-contrast edges: enable CA removal in RAW
Common mistake: Mistaking purple fringing for a sensor or focus problem.