The Dictionary Of Photography

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

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distance from the source of light, or, in other words, the greater the distance of the sensitive surface from the lens the longer the
Fig. 58.
exposure. This is very clearly seen from the above diagram. Let L be the source of light, and if we place the bromide paper at 1., 12 ins. from the light, and we find the exposure to be 45 seconds, when we place the paper at 11., III., iv., respectively - i.e., at 24, 36, and 48 ins., the exposure will not be 45, 90, 135, and 180 seconds", but in the proportion of 45ยป J8o, 395, and 720 seconds respectively. The above law is not absolutely true for actual sources of light, which necessarily have some magnitude, but the law is a good practical guide where the source of light is small.
(5) The Sensitiveness of the Material on which the Enlarge-ment is made. This has been, to a great extent, a matter of conjecture hitherto; but Mr. Alf. Watkins publishes tabular statements from time to time for use with his exposure meter.
Finding Exposure by trial. Many persons will find that, owing to the many factors involved in calculations for exposure in the case of enlargements, it is convenient to determine the exposure by preliminary trial on a narrow strip of the paper to be used. All being adjusted, a strip of the paper is fixed in such a position that it crosses characteristic parts of the image, and a trial exposure is given; but often several trial exposures can be made on the same strip of paper - the strip being un-covered by stages. The first trial will often give a sufficient indication as to the right exposure, but the trial method may be repeated until something like certainty is reached. Even when an estimate or calculation has been made it is often worth while to confirm by a trial on a slip of paper.