Colour fringing
Simulator
AI generated demo image
Two different faults with the same name
Glass bends blue light more than red. A lens that does not correct for it draws each colour at a slightly different size, or at a slightly different distance, and the two give you quite different problems.
Lateral: wrong size
Red and blue are magnified by slightly different amounts, so they line up perfectly in the middle of the frame and separate towards the corners. It shows as a coloured outline on high-contrast edges, always pointing away from the centre.
Longitudinal: wrong distance
Each colour comes to focus at its own distance, so at any one focus setting some of them are soft. It shows as green fringing behind the focus and magenta in front of it, right across the frame including the middle.
Only one of them stops down
Narrowing the aperture deepens the focus until all the wavelengths are inside it, which kills longitudinal fringing. It does nothing to the magnification error, so lateral fringing is as bad at f/16 as at f/1.4.
Only one of them comes out in post
Lateral fringing is geometry: scale the red and blue channels back together and it vanishes, which is what a lens profile does. Longitudinal fringing is lost focus in one colour, and nothing can put that back.
Where it shows up
Backlit branches, chrome, water, spectacle frames, white shirts against a dark suit. Anywhere there is a hard light-to-dark edge, and especially out at the edges of frame where lateral error is largest.
What it costs to avoid
Low-dispersion and apochromatic elements are the expensive part of a lens. A fast zoom with visible fringing wide open is not a broken lens, it is a lens that chose speed and price over correction.