Lens Story: 11 of 28
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.
THE CAMERA AND ITS LENS
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.
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.
EXAGGERATION PRODUCED WITH|
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.
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.
CORRECT PERSPECTIVE OBTAINED|
WITH LONG-FOCUS LENS
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.