30 



HANDBOOK OF PHOTOGRAPHY 



loss of contrast, the effect being almost similar to admitting air&y light into the 

 camera by making holes in the bellows! 



Spherical aberration can be immediately recognized in a lens by letting it form an 

 image of a distant point source through a filter, to make the light reasonably mono- 

 chromatic, and then examining the image with a strong magnifier or a low-power 

 microscope. 



Coma. — Coma is an aberration which does not exist in the center of the image 

 (the lens axis) but increases steadily for images lying progressively farther out in the 



Fig. 24. — Image produced when coma is present. 



field. It is essentially a difference in magnification, or distance of the image from the 

 lens axis, for different zones of the lens. Each zone forms a minute ring image of an 

 object point, all the various rings falling between two lines at 60° to one another as 

 indicated in Fig. 24. The strongest concentration of light is, as for spherical aber- 

 ration, where the rays through the middle of the lens form their focus, viz., at P in 

 Fig. 24. This comatic (cometlike) form of star image off the axis of the lens is very 

 commonly found in an ordinary astronomical telescope having a slightly tilted 

 objective. It is common too in photographic lenses, but there it is generally so 



700 1- Red 



400 



■^Lighf_ 



Position of Focus 



direct ion 



Fig. 25. — Curve relating wavelength with position of focus, for a simple lens and for an 



achromat. 



mixed with astigmatism and other aberrations that few people have ever seen an 

 image afflicted with pure coma and nothing else. 



Chromatic Aberration.— This is characterized by a longitudinal displacement of 

 the image plane for different wavelengths (colors). In an uncorrected positive lens 

 the violet focus is the shortest and the red the longest, the other colors falling into 

 their place between the two extremes. In an achromatic lens an attempt is made by 

 combining two or more different kinds of glass to unite two colors at a common focus. 

 When this is done, the intermediate colors fall closer to the lens than the united pair, 



