The Dictionary Of Photography

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

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The formula determined for / can approximately be written as follows: -
For this the values of L, according to Dorval, are: -
"Day," in Summer, from 9 to 4; in Winter, from 11 to 2.
(2) The Plate. The sensitiveness of the plate used is another important factor, and is that factor concerning the accurate expression of which there is most difficulty ; and most workers - even at the present time - prefer to secure uniformity of results by keeping as far as possible to one make of plates, and if using another make to ascertain what allowance to make, once for all, as double, one and a third, or half. Various systems of plate-speed numbers have been devised, and the Hurter and Driffield system, p. 306, is based not on the minimum exposure required to obtain an impression on the plate, but rather on that required to produce a full gradation of tones throughout the scale, is at least as reliable as any other system. As several makes of commercial plates are now marked in accordance with the Hurter and Driffield system, and the actinograph is graduated to correspond, this is a very convenient system to adopt. And by