Chromatic aberration produces coloured fringes around the lens edges, and parts of the image may not be sharp [C]. This aberration occurs with single lenses because they behave like prisms and bend blue light more than red light [A]. Combining the lens with a weaker concave lens [B] made of a different glass cancels out this dispersion effect, and both red and blue rays are brought to the same focus to produce a sharper, more distinct image.
Chromatic aberration is a defect in a lens in which the various colors of the spectrum are not brought to the same focus. Blue light, for example, is refracted (see refraction) more than red light when it passes through a lens and hence comes to a focus inside that of red light. The result is fringing – the formation of a colored halo around the image. This problem seriously affected the performance of refracting telescopes for centuries and was the reason that so many refractors were built with large focal ratios: longer focal length lenses show less chromatic error. A better solution came with the introduction of corrective elements, using at least two different types of glass, into a compound lens. An achromatic lens corrects for red and blue light, whereas an apochromatic lens corrects for at least red, blue, and green. Reflecting telescopes are free from this type of aberration.