Submitted by Al …
(Adapted from The Encyclopedia of Photography ©1963)
A lens which is free from astigmatism and which focuses both vertical and horizontal lines without distortion, with equal brightness and definition, is known as an anastigmat. In such lenses, all the points in the scene are in exactly the proper place on the film or plate.
Astigmatism exists in lenses when the horizontal and vertical lines entering the lens cannot be focused in the same plane at the top and bottom or sides of the field. At the focus for horizontal lines, the image of a point is drawn out to a narrow ellipse lying horizontally; at the focus for vertical lines the same condition exists, but lies vertically.
The correction of this condition occupied early opticians for many years. First they attained the periscopic lens, composed of two meniscus lenses, one before and one behind the stop opening.
In order to increase the aperture of this lens, which was about f / 11, the opticians created the rapid rectilinear lens, which was corrected for spherical and chromatic aberrations. The rapid rectilinear, which requires that both lenses be compound, had an aperture of f / 8.
But none of these lenses were corrected for curvature of field or astigmatism. This required special glasses which were not available to the earlier opticians. When this special material was incorporated into the rapid rectilinear it brought the number of components up to six. Several different anastigmat lenses of this type were made, having an aperture of f / 6.8. In seeking to improve the definition or increase the aperture, opticians kept adding other components until each member contained four or even five lenses cemented together. Manufacture was made extremely difficult.
However, several opticians starting from scratch with the new glasses devised newer, unsymmetrical lens designs which simultaneously corrected all the earlier errors and astigmatism as well. Dr. Paul Rudolph several lenses, one of which contained only four elements. Two of these were cemented together and placed on one side of the diaphragm; the other two were separated by a small air space and placed on the opposite side of the diaphragm. This design was named the Tessar and is the basis for a great many modern lenses of speeds up to about f / 2.8.
In England, H.D. Taylor devised a different arrangement using only three lenses. The two outside elements were alike and were positive or magnifying elements. Midway between these was placed a single negative lens which carried the entire burden of correcting the system for all aberrations. These triplet lenses can be very well corrected for apertures to about f / 3 and are used in may low priced still cameras.
For lenses faster than f / 2.8, correction for spherical and chromatic aberration and astigmatism must still be carried out separately. These high speed lenses, often reaching apertures as high as f/ 1.5 and even faster, may contain as many as ten or more separate elements, some cemented and some separated by air spaces.
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