The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Diaphragms
give the fraction expressing relative rapidity ofor whatever it may be. Per-
sonally I have had a piece of thin brass cut exactly to measurement, and thus ruled and continued till at its widest part it measures four inches. By inserting this in a stop, one is able at once to read off the diameter, and, dividing the focal length by it, to obtain thenumber. The following
convenient diagram and table were given some years back in the " British Journal of Photography Almanack ": -
The Royal Photographic Society's older Standard for Diaphragms. - The annexed diagram and table are intended to facilitate the calculation of the proper number with which to mark the diaphragms according to the Royal Photographic Society's old or Uniform-System, which will be found de-scribed on another page. This number it was proposed to call the " U. S." (or uni-form system number), a system still of use to the amateur as a means of esti-mating and calculating. The numbered circles in the diagram represent the sizes of stops. The photographer, knowing the equivalent focus of his lens, looks along the line opposite the number which represents the circle nearest inside to his diaphragm, and when he gets to the column headed by that equivalent focus the number there found is the U.S. number to be marked on the diaphragm. For example, a lens of eight inches equi-valent focus has a diaphragm in size about No. 5 on the diaphragm. Running the eye along the line opposite No. 5, we find in the column under " focus eight inches" the number 11, which is the U. S. number required.
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