chromatic aberration

Chromatic aberration occurs because glass refracts different wavelengths of light at slightly different angles. Red light bends less than blue light when passing through a lens. This means a single lens cannot focus all colors at the same point, leaving colored fringes, typically red and cyan, around high-contrast edges in the image.

The effect scales with lens design and wavelength separation. A simple single-element lens shows severe fringing. Compound lenses with multiple elements of different glass types, called achromatic or apochromatic designs, minimize the problem by making the refraction index more uniform across visible wavelengths. Professional optics use expensive low-dispersion glass and complex multi-element designs to reduce aberration to acceptable levels for their application.

Chromatic aberration is worst at the edges of the field of view and becomes more visible at wide apertures. In telescopy and microscopy, it limits resolution and contrast. In photography, it becomes obvious in high-resolution digital cameras and when shooting high-contrast subjects like branches against sky. Wide-angle camera lenses historically showed it prominently; modern designs and digital correction have made it less noticeable in consumer gear.

The condition is named for the prismatic separation of color, not for any defect in the optical material itself. Manufacturers often list the Abbe number of their glass, a figure that directly indicates resistance to chromatic aberration: higher numbers mean lower dispersion. Standard borosilicate crown glass sits around 64; dense flint glass drops to 35 or lower, making flint useful only in specific positions within a compound lens.

Digital imaging systems can now apply software correction during post-processing, mapping color channels slightly to realign them. This works only when the pattern of aberration is known and consistent, making it most effective for cameras with fixed lenses rather than interchangeable ones.