The Dictionary Of Photography

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

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Diaphragms
We have stated above that to determine the value of the diaphragms it is necessary to find-the focal length and divide this by the aperture of the diaphragm. Burton gives a very convenient table in his useful little handbook, " Modern Photo-graphy," which gives the diameter of the stop aperture for lenses of any focus from 6 to 11 in., and the diameter of any aperture for lenses of less or longer focus may be found by multiplying or dividing, as the case may be. We here insert this useful table: -
Now for an example or two how to use this table. We have a lens of 5-in. focus, and want to cut our diaphragms for
Turning to the table, we find no 5-in. lens included, but we find a 10-in. ; therefore we have the diameters there given. Carrying the eye down the column under 10-in., we find oppositeso that we
must cut our apertures to
and so on. Where the focus of the lens is given in the table we merely take the apertures there given. To some it may be difficult to accurately measure the diameters of apertures, and to facilitate this we include a diagram (fig. 43) taken from the " American Annual of Photography." Each cross line varies in length from the adjacent one To use, lay the stop flat on this scale, and select a cross line which is of the same length as the greatest diameter of the opening; read this off by means of the figures, which will be the measurements The equivalent focal length
of lens, divided by this measurement of the stop opening, will
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