File:The American annual of photography (1912) (14579284808).jpg

Identifier: americanannualof1912newy (find matches)
Title: The American annual of photography
Year: 1912 (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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dpoint of simple compound numbersand also subject the lens manufacturers to a governmentalstandard in marking diaphragm scales and in designating theextreme speed of lenses. The writer once purchased a lensadvertised as F-7.7 and on account of the way it was mountedIt proved on investigation to have only a speed of F-9 or 73per cent, of the advertised speed. A quart cup brings onlya few cents in the market, but let a manfacturer cut one ofthese measures to only 73 per cent, of its legal volume andimagine what would happen. The fixing of two units suffices for working the system.One is a cone unit, which may be compared to our degreesfor the measurement of circles, except that it deals withvolume as represented by a sphere instead of with surfaceonly. For good reasons I have chosen the small cone F-64as the unit cone. As applied to lens stops this scale is simplythe unit values of the cones of light which the stop allows topass the lens in this scale. Stop 4, for example, will do a 220
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WATERFALL, DINGMAN FERRY, PA. JOSEPH LAURIER. certain work in a quarter the time required by stop i, justas 4 men would do a certain job of work in a quarter thetime that one would do it. The following table will show the unit scale as comparedwith both the Uniform and the F systems of stops:U. system..256 128 64 32 16 8 4 2 i F system... 64 45 32 22 16 11 8 5.6 4Unit system i 2 4 8 16 32 64 128 256The other unit is that of the actinicity of surfaces. Inmaking this measurement the light from a surface is im-pinged upon a strip of ordinary film through an opening hav-ing the value of F-i or 4096 cone units, in such a way as totint the outline of a small hole on the emulsion when it is ex-posed to a surface. A surface has one unit of actinicity whenone minute (or 64 seconds) is required to create a least visi-ble tint on the surface of the film, the strip being removedfrom contact with the instrument while examining it. It is clearthen that if 32 seconds sufficed to get the faintest tin

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