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The object of this table is to enable any manipulator who is about
to enlarge (or reduce) a copy any given number of times to do so
without troublesome calculation. It is assumed that the photographer
knows exactly what the focus of his lens is, and that he is able to
measure accurately from its optical center. The use of the table
will be seen from the following illustration: A photographer has a
_carte_ to enlarge to four times its size, and the lens he intends
employing is one of 6 inches equivalent focus. He must therefore
look for 4 on the upper horizontal line and for 6 in the first
vertical column, and carry his eye to where these two join, which
will be at 30-7-1/2. The greater of these is the distance the
sensitive plate must be from the center of the lens; and the lesser,
the distance of the picture to be copied.

In practice it is convenient, after having once found the focus for a
given enlargement from a given negative with the lens in use, to mark on
the base of the apparatus the point to which the lens has been extended.
Then in making future enlargements of the same size, it is only necessary
to set the lens at that point and move the easel backward or forward until
an approximate focus is obtained, when the image will be of the proper
size on the screen.

As an approximate guide it is sufficient to know that the nearer the lens
is to the negative the greater will be the enlargement, as may be seen in
Fig. 7. If a piece of thin cardboard, or a sheet of paper cut to the exact
size of the enlargement desired, is placed upon the easel-screen, little
difficulty will be experienced in getting an enlarged image of the proper
size and correctly focused.

It is advisable to focus the enlargement with the largest aperture of the
lens. If the lens, when working at its largest aperture, covers the plate
from which the enlargement is being made, it will give proper definition
over the enlargement. With a lens of the better sort, of course, the
definition will be equally good whether a large or small aperture is used;
but with a low-priced lens it is better to stop down to No. 8 (f/11.3) or
No. 16 (f/16), to avoid spherical aberration. Stopping the lens down
increases the time of exposure, and enables one to have greater control
over the operation of exposing the paper, permitting time to shade or
locally increase the exposure at an