What is chromatic aberration in optical lenses?

Chromatic aberration in optical lenses is the failure of a lens to focus all colours to the same point.

Chromatic aberration, also known as "colour fringing" or "purple fringing", is a common optical problem that occurs when a lens is either unable to bring all wavelengths of colour to the same focal plane, or when wavelengths of colour are focused at different positions in the focal plane. This is because lenses have a different refractive index for different wavelengths of light. The refractive index of transparent materials decreases with increasing wavelength. In other words, blue light is bent more than red light. This results in the image being blurred or having noticeable coloured edges (fringes).

This phenomenon can be particularly noticeable in photographic images, where it can cause coloured fringes on high-contrast edges within the picture, and can be especially problematic in high-resolution imaging systems. It's often seen as a problem in cheaper lenses, as higher-quality lenses use special types of glass and lens coatings to minimise the effects of chromatic aberration.

There are two types of chromatic aberration: longitudinal (or axial) and lateral (or transverse). Longitudinal chromatic aberration occurs when different colours come into focus in front of and behind the focal plane. Lateral chromatic aberration occurs when different colours come into focus at different positions on the focal plane because the magnification and hence the position of the focal plane is colour dependent.

To correct chromatic aberration, lens makers often use multiple elements made from different types of glass. These are designed to bring two specific wavelengths into focus in the same plane. This is known as an achromatic lens. However, even with an achromatic lens, other wavelengths are still not focused perfectly, leading to residual chromatic aberration. This can be further reduced with an apochromatic lens, which is designed to bring three wavelengths into focus in the same plane.