Chromatic aberration

Colour aberration and its correction.

In optics, chromatic aberration is a type of optical distortion caused by the inability of a lens to focus all colors on a single point of convergence.

The focal length of a lens depends on its composition (which determines its refractive index) and its shape (the geometry of its surfaces). Since the refractive index of all optical substances varies with wavelength, the focal length of a lens differs for each color. Consequently, a lens does not form a single image of the object, but rather a series of images (one for each color present in the incident light), each with its own focal length. Furthermore, since the size of the resulting image depends on the focal length itself, these images also have different sizes from each other. Longitudinal chromatic aberration is the variation in distance with the refractive index, and lateral chromatic aberration is the variation in image size.

Light of shorter wavelength (blue) is bent more than light of longer wavelength (red), so blue light is focused at a point closer to the lens than light red. The effect can be reduced by placing two lenses together in a configuration known as a pair, pair, or achromatic doublet. It should be noted that mirrors do not suffer from chromatic aberration, since light is simply reflected without changing the medium (ie, there is no difference in refractive index).

At the top there is detail in the corners because it is a photograph taken with a high quality objective. At the bottom there is a similar photograph taken with a wide angle lens showing an obvious chromatic aberration...

In practice, longitudinal chromatic aberration is understood as the effect of colored edges that occurs around an object seen through a lens, caused by the inability of the lens to deflect all colors towards the same focus.

Lateral chromatic aberration (SMC, multi-layer system) results in a higher proportion of white in the image. It usually happens when you don't use a sunshade.

In the same way that there are longitudinal and lateral chromatic aberrations, in practice we can find achromatic aberrations, which are easily identifiable because the variation of the powers of each pair of glued lenses (doublets) must be equal to 0. ^Doubtful If the glued doublet is made up of two sufficiently thin lenses, the doublet as a whole can be considered to be thin enough. In such a case, the achromatism condition entails a simultaneous correction of positional chromatic aberration (PCA) and magnification chromatic aberration (ACA). However, a glued doublet consisting of two thick lenses cannot correct ACA and ACP simultaneously. This is because the position of the principal planes of the optical system varies with the wavelength.