File:The American annual of photography (1922) (14801618453).jpg

Identifier: americanannualof36newy (find matches)
Title: The American annual of photography
Year: 1922 (1920s)
Authors:
Subjects: Photography
Publisher: New York : Tennant and Ward
Contributing Library: Harold B. Lee Library
Digitizing Sponsor: Brigham Young University

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al aberration (of direct pencils) has beensacrificed to attain marginal definition, speed or both. Distortion was fully covered in the 1921 Annual, so that nofurther mention need be made of it here. Astigmatism and coma have long been the bete noir of goodlenses, and they are most difficult to explain because no draw-ing can be made to properly illustrate them. Paul L. Anderson in his book Pictorial Photography givesperhaps the best definition and illustration of astigmatism inprint today. We quote: Astigmatism is that defect in which bundles of rays pass-ing obliquely through the lens near the margin are converged,not to points, but to straight lines. Each such bundle is con-verged to two straight lines at different distances from the lensand at right angles to each other, or, rather, it is convergedto a line, and after passing the point of convergence it di-verges again to another line. Astigmatism is difficult to illus-trate graphically, but may be understood by anyone who will 248
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AN OLD FASHIONED GIRL. William Alexander Alcock. roll up a truncated cone of paper and pinch the small end to astraight line, afterward pinching the cone to another straightline at right angles to the first and at some distance from theend. The straight lines represent the projection of a point ofthe object, whereas if the lens were free from astigmatism thisprojection would be represented by rolling the paper so as toform a complete cone, the small end being a true point. Itshould be noted that astigmatism appears only at the marginsof the field, so will not be noticeable if the lens is of greatfocal length relatively to the plate used. The practical resultof astigmatism is to render it impossible to focus simultane-ously on vertical and horizontal lines in the same plane andlying near the edges of the field, so that if the image of atree, for instance, comes close to the edge of the plate thetrunk will be sharp and the branches blurred, or vice versa,but it will not be possible to g

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