Spherical aberration

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Spherical aberration. A perfect lens (top) focuses all incoming rays to a point on the optic axis. A real lens with spherical surfaces (bottom) suffers from spherical aberration: it focuses rays more tightly if they enter it far from the optic axis than if they enter closer to the axis. It therefore does not produce a perfect focal point. (Drawing is exaggerated.)

A point source as imaged by a system with negative (top), zero (centre), and positive (bottom) spherical aberration. Images to the left are defocused toward the inside, images on the right toward the outside.

Longitudinal sections through a focused beam with (top) negative, (centre) zero, and (bottom) positive spherical aberration. The lens is to the left.

Spherical aberration is an optical effect observed in an optical device (lens, mirror, etc.) that occurs due to the increased refraction of light rays when they strike a lens or a reflection of light rays when they strike a mirror near its edge, in comparison with those that strike nearer the centre. It signifies a deviation of the device from the norm, i.e., it results in an imperfection of the produced image. Spherical aberration should not be confused with barrel distortion; whereas barrel distortion affects the shape of the image, spherical aberration affects its sharpness.

A sphere lens has an aplanatic point (i.e., no spherical aberration) only at a radius that equals the radius of the sphere divided by the index of refraction, that is inside the lens. It is often considered to be an imperfection of telescopes and other instruments which makes their focusing less than ideal due to the spherical shape of lenses and mirrors. This is an important effect, as spherical shapes are much easier to produce than aspherical and so most lenses have spherical shapes.

"Positive" spherical aberration means peripheral rays are bent too much. "Negative" spherical aberration means peripheral rays are not bent enough.

The effect is proportional to the fourth power of the diameter and inversely proportional to the third power of the focal length, so it is much more pronounced at short focal ratios, i.e., "fast" lenses.

For small telescopes using spherical mirrors with focal ratios shorter than f/10, light from a distant point source (such as a star) is not all focused at the same point. Particularly, light striking the inner part of the mirror focuses farther from the mirror than light striking the outer part. As a result the image cannot be focused as sharply as if the aberration were not present. Because of spherical aberration, telescopes shorter than f/10 are usually made with non-spherical mirrors or with correcting lenses.

In lens systems, the effect can be minimized using special combinations of convex and concave lenses, as well as using aspheric lenses.

For simple designs one can sometimes calculate parameters that minimize spherical aberration. For example, in a design consisting of a single lens with spherical surfaces and a given object distance o, image distance i, and refractive index n, one can minimize spherical aberration by adjusting the radii of curvature R₁ and R₂ of the front and back surfaces of the lens such that

.

Spherical aberration of collimated light incident on a concave spherical mirror.

See also

• Aberration in optical systems
• Hubble Space Telescope
• Maksutov telescope
• Parabolic reflector
• Ritchey-Chrétien telescope
• Schmidt corrector plate
• Soft focus

External links

• Spherical aberration at vanwalree.com, PA van Walree, viewed 28 January 2007.
• http://www.telescope-optics.net/spherical1.htm
• "http://doflick.com/ViewVideo.aspx?vId=284" A clear video explanation of Spherical aberration and hyperbolic lenses