Aberration and other light distortions

Abstract light art, example of aberration
Source: Unsplash

That’s no moon! That’s… a distortion of our specimen caused by a phenomenon called aberration. (Is it safe to assume the Venn diagram of Star Wars fans and microscope enthusiasts is a circle?)

Aberration, in optics, is the deviation of light rays through lenses, causing images of objects to be blurred. Normally, every point on the object will focus to a point of zero size on the image, but aberration causes light to be spread out over some region of space rather than focused to a point. The result is that images of the specimen are blurred or distorted. How they are distorted and to what degree vary depending on the type of aberration, of which there are several.

Spherical

In spherical aberration, rays of light from a point on the optical axis of a lens having spherical surfaces do not all meet at the same image point. Rays passing through the lens close to its centre are focused farther away than rays passing through a circular zone near its rim.

For every cone of rays from an axial object point meeting the lens, there is a cone of rays that converges to form an image point, the cone being different in length according to the diameter of the circular zone. Wherever a plane at right angles to the optical axis is made to intersect a cone, the rays will form a circular cross section.

Coma or Comatic Aberration

Coma aberration refers to aberration inherent to certain optical designs or due to imperfection in the lens or other components that results in off-axis point sources such as stars appearing distorted, appearing to have a tail (coma) like a comet. Specifically, coma is defined as a variation in magnification over the entrance pupil. The usual way for reducing coma is to employ a diaphragm to eliminate the outer cones of rays.

Chromatic

Chromatic aberration is the failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the lens elements varies with the wavelength of light. The refractive index of most transparent materials decreases with increasing wavelength. Since the focal length of a lens depends on the refractive index, this variation in refractive index affects focusing. Chromatic aberration manifests itself as “fringes” of color along boundaries that separate dark and bright parts of the image.

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