No. 256, |1-1/2 | 5-1/2 | 32 |42 0 |42 0| 8 30 | 42 | 4 15|17 0
or f/64 | | | | | | | | |
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The estimation of this correct exposure is probably the greatest
difficulty in photography, and it is particularly discouraging to find
plate after plate useless because the guess has been wide of the mark.
There are some here to-night who have spoiled so many plates that at last
they are prepared by experience for almost any contingency, and to those
I nave very little to say; but there are also many who are still in their
troubles, and I propose to tell them how the amount of guesswork required
may be reduced to a minimum.

The factors which govern exposure are: the subject of the picture, the
lens and its aperture, the rapidity of the plate, and last, but not by
any means least, the quality of the light by which the work is to be
done.

Let us consider each of these separately, and see if we cannot reduce any
of them to rule. In this respect the subject will be found somewhat
intractable. Scarcely two subjects will be found to send exactly the same
amount of light through the lens. However, a broad classification may be
made, and this has been done by Mr. Burton in his Table of Comparative
Exposures. A glance at this table will show how greatly the character of
the view may influence the time of exposure. Thus, with full aperture of
a rapid symmetrical, the exposure for open landscape is given as
one-twelfth of a second; when heavy foliage appears in the foreground,
half a second will be required; while, under trees, as much as forty
seconds may be needed.