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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