Popular Science Monthly 



5C 



C 



manufacturer is faced with one insurmount- 

 able difficulty — in grinding photographic 

 lenses he can produce only spherical sur- 

 faces. He can't grind glass into parabolic 

 curves; the basic principle in grinding 

 precludes that. So the lens maker, to 

 overcome these difficulties, has to use 

 several kinds of glass and to combine the 

 lenses made from these various kinds of 

 glass so to make the finished photographic 

 objective. He grinds some glasses with 

 deep concave cur\'es, some with convex 

 curves. Wherever he 

 can get two curves that 

 will fit — one being con- 

 cave and one convex — 

 he cements the glass 

 surfaces together. And 

 so he persuades the 

 lens to carry out his 

 primary object and 

 bring together at a 

 point on the film the 

 different light rays re- 

 flected from the same 

 point in the object. 



The genesis of ever\- 

 lens is a design on 

 paper. The lens is cal- 

 culated by a lens-com- 

 puter, who is as much 

 at home with abstract 

 mathematics as a baker 

 at his doughboard. 

 After he has computed 

 the lens, he passes it on 

 to the workman, who 

 grinds and assembles 

 it exactly according to 

 the calculation. The 

 computer then sub- 

 mits his lens to test on 

 the lens bench. On 

 the other side of the 

 room is a tiny pin- 

 point of light, very 

 brilliant, and so ar- 



inged that its color 

 * an be varied. By this 

 means the bench can 



'St a lens not only for 



rdinary light, but for 

 red and green and yel- 

 low and blue light as 

 well. 



The lens is put in its 

 holder on the top of its 

 own column (see illus- 

 tration on preceding 



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Illustrations of the p>oint of light. 

 The strip at left shows a sharp 

 picture in the middle of the 

 film but blurred ends. The 

 center strip shows astigmatism, 

 the lines changing direction. At 

 right is the blurring called coma 



page). It forms a tiny image of the point 

 of light across the room, which point of 

 light is examined by the microscope. At 

 the end of the microscope a little camera is 

 placed, in which is a strip of motion-picture 

 film. On this strip of film, pictures are 

 taken of the point of light. 



The vertical strip at the left in the illus- 

 tration below shows the sharpness of the 

 image that the lens will give in the middle 

 of the picture when the light is going 

 straight through the center of the lens. 

 But at the edge of a 

 picture the light is not 

 going straight but side- 

 ways through the lens, 

 and the real test of a 

 photographic lens is 

 the . sharpness of the 

 edges of a picture. 

 Nearly all lenses will 

 give a sharp picture in 

 the middle of the film, 

 but only a really good 

 one will give a sharp 

 image at the edges. 



Sometimes instead of 

 getting a point image 

 at the edge we get a 

 short line which 

 changes its direction 

 according to the exact 

 focus of the lens, the 

 line being vertical in 

 one position and hori- 

 zontal in another. (See 

 middle strip in illus- 

 tration.) This defect 

 is called astigmatism. 

 An even more curious 

 defect is the blurring 

 known as "coma," the 

 point being shown as a 

 dot with luminous 

 wings like those of an 

 insect. (Vertical strip 

 at right of illustration.) 

 The lens-testing 

 bench measures ex- 

 actly all the devia- 

 tions of the lens from 

 its theoretical perfec- 

 tion. Armed with this 

 knowledge, the lens- 

 computer can draw his 

 curves and alter his 

 construction until the 

 lenses measure up to 

 the highest standard. 



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