File:The American annual of photography (1918) (14781285865).jpg

Identifier: americanannualof1918newy (find matches)
Title: The American annual of photography
Year: 1918 (1910s)
Authors:
Subjects: Photography
Publisher: New York : Tennant and Ward
Contributing Library: Harold B. Lee Library
Digitizing Sponsor: Brigham Young University

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Figure 1. tract the third distance from the second and the remaindershould tally with the first result. As an example, suppose when the rule is focused half-size, so as to be exactly 6 ins. long on the screen, the distanceis i8i^ ins., as at A in Figure i ; while when the rule is one-third size, measuring 4 ins. on the screen, the distance is 24^ins., as at B. Then, 18^ ins. from 24^ ins. leaves 6 ins.,which is the focal length. Similarly if, to verify accuracy,the rule is focused to 3 ins. long, or one-fourth size, the dis-tance, providing all has been done correctly, will prove to be30J4 ins., and 24^ ins. from 30^ again leaves 6. Should a long bench be available, the camera may be placed 212 \
Text Appearing After Image:
m <<< < w 213 on this, and the rule secured to an upright easel fixed at theother end. Then, by marking the bench in inches, commenc-ing from the rule, or attaching a paper scale, the distances arevery easily read off. It may be explained that the odd half-inch in all supposed measurements just quoted is due to tak-ing the distance from the lens-board, instead of from the nodeof admission of the lens itself. This does not affect the result,since it appears equally in each measurement, and it is only thedifference between them that is wanted. This brings us to a second common difficulty. In copy-ing or enlarging to a definite size, the photographer oftenwishes to have the lens a certain distance from the plate, butdoes not know from what part of the lens to measure. Thetext-books say all such measurements should be taken fromthe node of emergence, but the description of the latter isseldom such as to enlighten the non-mathematical reader. Inreality, stripped of lea

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