Spherical Aberration

A lens is said to have an aberration if it does not produce a point image. In practice, lenses that are not spherical do not produce point images. Spherical aberration is the term used to describe the formation of an image by a spherical lens will not be perfect because rays striking regions near the perimeter of the lens will be brought to a slightly different focus than those striking the inner regions.

In general a point object will form an image that is a small circle of light. There are several ways of reducing this effect.

The shape of the lens could be change to sharpen the image. The lens would no longer be spherical. This only works for objects a certain distance from the lens.

The aperture can be reduced by use of a diaphragm as shown below.

The amount of light forming the image is reduced and diffraction effects are introduced.

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