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Page:Popular Science Monthly Volume 90.djvu/74

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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

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