What the lens is doing wrong
A simple lens bends blue light more sharply than red, in the same way a prism splits white light into a spectrum. That means a single piece of glass cannot bring every colour to focus at the same place, and the residual disagreement appears as coloured edges.
Optical designers spend most of their effort on this. Achromatic doublets pair glass types with opposite dispersion to cancel it, and modern apochromatic designs push the correction across three wavelengths, which is why the word apo on a lens barrel commands a price. No design removes it completely.
The two forms behave differently and it is worth being able to tell them apart. Longitudinal aberration is a focus error, so it shows up as magenta or green haloing around out-of-focus edges everywhere in the frame, including the centre. Lateral aberration is a magnification error, so the frame centre is clean and fringing grows steadily toward the corners.
Why generated images want it back
An image with perfectly registered colour channels has a specific quality: it looks synthetic. There is no such thing as a real photograph without some lens signature, so the absence of one is itself a cue.
This makes a small amount of chromatic aberration part of a family of deliberate imperfections used to make rendered and generated frames convincing, alongside grain, vignetting, slight softness in the corners and halation. Each one is a piece of evidence that light passed through glass onto a sensor.
The discipline is restraint. Fringing that a viewer notices has stopped being evidence and become an effect. Real colour fringing:
- Sits at roughly one pixel wide at normal viewing size.
- Appears only where contrast is genuinely extreme.
- Is absent from flat mid-tone areas.
- Grows toward the corners if it is lateral, or surrounds defocused edges if it is longitudinal.
Prompting and grading it
Models respond to the term itself, but they overshoot, because the strongest training examples are heavily processed images where the effect was pushed as a style.
Qualify the strength in the prompt: slight chromatic aberration, faint magenta and cyan fringing. Then say where it should appear, since that is what separates optical behaviour from a filter: on the high-contrast edges of her shoulders and hair.
Naming the glass often works better than naming the defect. vintage uncoated lens character, fast 50mm wide open, old Soviet lens all bring the fringing along with the rest of a coherent lens signature, and the result is more plausible than asking for the aberration alone.
If you are adding it in post rather than in the prompt, do it by scaling the red and blue channels apart by a fraction of a percent around the frame centre, which reproduces lateral behaviour correctly. Applying a uniform offset across the whole frame is the common mistake, because it puts fringing in the centre where a real lens has almost none, and that is exactly what makes an image look filtered rather than photographed.