The Different Kinds of Lens Defects

By Daniel Zimmermann
Microscopes must make adjustments for lens defects.

Convex lenses have played important roles in scientific discovery. Telescopes have enabled scientists to view distant celestial bodies. With microscopes, scientists have discovered the basic constituents of life. Through the camera, explorers have acquired a permanent record of their discoveries in the natural world. The convex lens is the chief component of these three instruments. Although dependable, the convex lens has intrinsic defects with which instrument makers have had to deal.

Construction and Function

A double convex lens is a disc-shaped object made of materials such as glass or plastic. If properly constructed, each of the two sides of this disc will bulge out in a regular curve to form a section of a sphere. When parallel rays of light impinge upon this lens perpendicular to the plane of the disc, the lens will refract or bend these light rays so that they come to a focus. A lens which effectively focuses light forms clear images and aptly fulfills its appointed role in a telescope, microscope or camera. However, if the lens has defects of construction, such as improper curvature or material that is not perfectly homogeneous, the images will proportionately suffer.

Spherical Aberration

Light impinging on different areas of the spherical surface of the lens will not meet at precisely the same spot. The rays striking the lens farthest from the center will focus slightly closer to the lens than rays than strike the lens near its center. This intrinsic defect of spherical lenses, called spherical aberration, results in a blurred image. Blocking the edge of a lens produces a better focus. In many instruments, a skillful combination of different lenses nearly eliminates spherical aberration.

Chromatic Aberration

Chromatic aberration results from the fact that a lens refracts or bends some colors of light more sharply than others. A lens bends violet light rays more sharply than green, and red suffers even less refraction. As a result, the lens tends to separate white light into its component colors, and a colorful halo results. The Englishman John Dollond solved the problem by the invention of the achromatic doublet, a combination of two lenses of different glass materials in which one type of glass corrected the chromatic aberration of the other.

Comatic Aberration

Comatic aberration occurs when light rays from a distance impinge upon a lens at an angle rather than perpendicular to the plane of its disc. The result is a comet-like figure with a tail. Proper grinding of the lens eliminates this problem. The term “chromatic aberration” comes from the word “coma,” which denotes the brilliant ball that surrounds the nucleus of a comet.