Page:Popular Science Monthly Volume 90.djvu/74

From Wikisource
Jump to navigation Jump to search
This page needs to be proofread.


Testing Your Camera Lens

How photographic lenses are given the third degree to unveil possible serious faults

���The lens is put in its holder on top of its own column. At the end of the microscope is a camera in which is a strip of motion-picture film for taking pictures of a point of light

��THERE are several v/ays to "try" lenses — and many devices for the purpose. A camera is only as good as Its lens, and the more severe the manu- facturer is toward his lens children, the better.

When the lens-tester illustrated here gets through with a lens, he knows all about it. Its innermost secrets are all revealed. If a lens shows partiality to certain colors, which it should not, the tester finds it out. If the lens is sufTering from astigmatism, the tester will show it. If it casts a drunken image, it has no chance of escape.

Next to a rneteorite, the bench for test- ing lenses i« about the heaviest thing for its size in existence. One is impressed, there- fore, when he discovers that it is built to hold a small microscope and a wee little lens about as heavy as a wet feather.

Why should such an elephantine machine be necessary to test lenses? Because the test must be made without vibration, and the distance between lens and microscope

��must be known within one ten-thousandth part of an inch. Perhaps the heavy parts now begin to explain themselves.

Before we describe this method of lens testing used in the kodak research labora- tory, it will be well to review the lens from its beginning.

If you have ever forced a stick into a clear pool and observed how curiously the reflection is bent, you have the working analogy of what a lens does to light rays. Refraction, as it is called, is that property of a substance which bends light rays, or, to be more accurate, which changes their direction. In the case of a lens, rays are bent so as to converge at a common point known as the focus.

Simply stated, the problem is to grind glass into the proper curvature so as to focus rays at a given point. But the implied simplicity of this statement is most mis- leading. The difficulties that beset this curve grinding are legion.

At the very outset in lens-making the

��58

�� �