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HANDBOOK OF PHOTOGRAPHY 



a symmetrical construction are generally found to have very little if any distortion. 

 Distortionless lenses are called "orthoscopic" or "rectilinear." 



Astigmatism. — This aberration, like coma, does not exist on the axis of a well- 

 centered lens but increases rapidly in the oblique pencils. It is characterized by a 

 longitudinal difference in position between the images of radial lines in the field and 



Fig. 27. — An astigmatic image. 



tangential lines. Thus, if a wheel having a tangential rim and radial spokes is photo- 

 graphed the spokes may be in focus and the rim blurred, or vice versa (Fig. 27). In 

 any oblique pencil through the lens from an extraaxial object point, the imaging beam 

 nowhere contracts to a point when astigmatism is present (hence the name), but 

 instead it shrinks to a pair of focal lines. Thus a series of sections across such a 

 beam would appear as indicated in Fig. 28 if shown side by side. In this figure, the 



e-K 



Fig. 28. — Diagram illustrating the images which are obtained when astigmatism is present. 



focal line marked R points radially in toward the center of the picture, and hence 

 radial lines in the picture would be imaged sharplj^ there. The other focal line T is 

 tangential to the picture, and tangential lines in the image are sharply defined there. 

 Midway between the focal lines the beam has a circular section, which represents 

 generally the position of best average definition. 



