Why a sharp edge can split into colors
A camera lens bends light to form an image, but glass does not bend every color by exactly the same amount. Because violet and blue light generally refract more than red light, the colors can land in slightly different places instead of meeting at one crisp point. At a dark branch against a bright sky, that tiny mismatch becomes a visible colored rim.
The pattern depends on the lens’s error. If colors focus at different distances along the optical path, the fringe can change with focus and may show up as purple in one area and green in another. If colors spread apart toward the edge of the frame, the colored outlines tend to grow farther from the center.
How lenses keep the colors together
Lens designers can pair elements made from different kinds of glass. One element bends colors apart; another is chosen to bring them closer together again. This correction adds design complexity, and controlling color fringing can be harder in inexpensive lenses, though price alone does not determine how much aberration a lens shows.
Cameras and editing software can also reduce some fringes. They detect color displacement near edges and shift color channels back into alignment. That works best when the mismatch is predictable; it cannot always restore detail that was blurred or recorded in the wrong place.
The effect is most noticeable where a bright area meets a dark one, so thin branches, lettering, and metal against the sky make useful checks. A colored edge there is not necessarily a flaw in the subject—it can be a sign that the lens has not brought all the image’s colors together.
