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 any portion of the image. This is sometimes useful, but as a general thing stopping the lens down is not advisable, as interfering with one's judgment in calculating exposures for various negatives. Having secured the image correct in size and focus, place thumb-tacks at all four sides of the sheet of paper or card used to focus the image. These will serve as a guide to the placing of the sensitive paper. Adjust the lens stop as desired and cap the lens, leaving the room totally dark save for such safe light as we may have for working. Place the bromide paper on the screen, using the thumb-tacks as a guide to the correct position in this.