The Dictionary Of Photography
A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.
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Diaphragms |
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ftosure, also the flatter the field of the le?is, and the greater the depth of focus. It is customary to give the apertures of diaphragms definite diameters; that is to say, the diameter of the diaphragm apertures should be a definite fraction of the focal length of the lens. There are several methods adopted, the one in general use being the ratio aperture, or
 system. To find this number divide the focal length of the lens by the diameter of diaphragm - e.g., focal length of lens, ins.; diameter of diaphragm, number of diaphragm, The Royal Photographic Society, in the year 1881, put forward a system for numbering the diaphragms in rather a different way, taking
 as the standard, which they call No. I. This system was termed the " Uniform Standard," or U. S. No., and the U. S. number for any diaphragm marked on the system may be found by the following rule: - Divide the focal length of lens by diameter of the diaphragm, and then square the result, divide by sixteen, and the quotient will be the U. S. No. Ex.: Find U. S. No. of diaphragm marked  or practically 8, U. S. No. In 1882 Dr. Stolze suggested a system of diaphragm numbering which was found by squaring the focal length and dividing this by the diameter of the diaphragm squared ; thus, taking as an example, a lens of focus and diaphragm apertures of we should find the numbers as follows: -
 In 1886 Dr. Stolze suggested, as an improvement on this, the marking of the stops with numbers, obtained by dividing the square of the focal length by one hundred times the square of the aperture, e.g.: -
 This practically means taking
 as the unit, which aperturewas also adopted by the International Congress on Photography, held at Paris in 1889. Mr. T. R. Dallmeyer recommended the
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