chromatic

Chromatic aberration occurs when a lens bends different wavelengths of light by different amounts, causing colours to separate instead of converging at a single focal point. A simple lens acts like a prism: blue light bends more sharply than red light passing through the same glass, so they focus at different distances from the lens. This colour fringing is called chromatic dispersion, and it degrades image sharpness across the visible spectrum and into the near-infrared and ultraviolet regions.

The root cause is the refractive index of optical glass itself, which varies with wavelength. Crown glass, commonly used in achromatic doublets, has a lower dispersion than flint glass, but neither eliminates the problem alone. A typical singlet lens shows lateral colour shift of 10 to 50 micrometres across the visible range, enough to blur fine detail in photography, microscopy, and precision optics. The effect worsens at the edges of the field of view and increases with lens aperture.

Correcting chromatic aberration

Optical designers combat this using achromatic and apochromatic compound lenses. An achromat pairs a positive crown glass element with a negative flint glass element, chosen so their dispersions partly cancel; this brings two wavelengths (typically red and blue) into focus at the same plane. An apochromat uses three or more elements and brings three or more wavelengths into alignment, crucial for colour-critical work like astronomical imaging or colour separation in reprographics.

Modern approaches include index-matched lens coatings and exotic materials like fluorite (calcium fluoride) and low-dispersion special glasses, which have flatter dispersion curves. Digital correction in post-processing can also reduce lateral chromatic aberration if the pattern is known, but this cannot recover lost information. In high-end telescopes, microscope objectives, and cinema lenses, chromatic aberration is measured in parts per million and specified as a design performance metric.

The term chromatic in optics always refers to wavelength-dependent behaviour; it is distinct from spherical aberration, which affects all wavelengths equally but results from the curved surface geometry. Quality optical systems must be corrected for both to achieve diffraction-limited performance across the working spectrum.