The Mentor · The Story of the Lens · Page 9

object is focused behind the retina because the crystalline lens is too flat or the eyeball is too short. To bring the image up onto the retina a far-sighted person holds objects off at a distance. In this case a lens that will converge the rays of light and bring them to focus earlier must be used. Spectacles with convex lenses will correct this defect.
Astigmatism is due to the fact that the refractive power of the eye is different in different planes. This results in more distinct vision in some directions than in others. A star, for example, is seen as a line of light. If a series of intersecting lines at all angles are drawn the astigmatic eye will see distinctly only those in a certain plane, the others appearing broad and blurred. This defect is corrected by wearing cylindrical glasses so ground as to reinforce the power of the eye in the plane of least curvature, or to neutralize the excessive curvature of the opposite plane.

EXAGGERATION PRODUCED WITH
SHORT-FOCUS LENS

The Camera Aside from the use of lenses in spectacles, no other adaptation has become so universal as that of the camera. But just as the telescope objective passed through a long period of evolution, so has the camera lens. Under a succession of masters of optics and scientific glass makers lenses to meet every need of the amateur and professional photographer have been perfected. Curvatures have been calculated with mathematical precision, and glass has been balanced against glass and element against element until every distortion and aberration has been compensated for.
One of the chief differences in camera lenses is length of focus. The longer the focal length the larger the image and the better the perspective. With a six-inch lens the image will be twice as long as it will with a three-inch lens. It is a matter of common experience, too, that most objects have a more pleasing perspective when seen from a distance.

CORRECT PERSPECTIVE OBTAINED
WITH LONG-FOCUS LENS

With a long focus lens, however, the field of view is more limited. Likewise pictures taken with a long focus lens are in true perspective, whereas if a short focus lens is used there is serious distortion.
Just as the telescope objective had to be corrected for the unequal refraction of light of different colors, so, too, did the camera lens. The result was the single achromatic meniscus lens followed by a combination of two such lenses with the stop between the two. This latter lens is called a "rapid rectilinear" lens because it gives straight line images and corrects the curvatures produced in the lines of an image taken with the single achromatic. When, toward the close of the last century, new and better kinds of optical glass were made the anastigmatic lens was produced. Unlike its predecessors, this lens will not only bring all the colors to the same focus, but it will also give flat field images in place of the saucer-shaped ones of the "old" achromats.