This section is from the "Henley's Twentieth Century Formulas Recipes Processes" encyclopedia, by Norman W. Henley and others.
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 30-7.5. 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. To reduce a picture any given number of times, the same method must be followed; but in this case the greater number will represent the distance between the lens and the picture to be copied, the latter that between the lens and the sensitive plate. This explanation will be sufficient for every case of enlargement or reduction.
Times of Enlargement and Reduction
If the focus of the lens be 12 inches, as this number is not in the column of focal lengths, look out for 6 in this column and multiply by 2, and so on with any other numbers.
To make a good enlargement five points should be kept constantly in view, viz.:
1. Most careful treatment of the original negative.
2. Making a diapositive complete in all its parts.
3. Scrupulous consideration of the size of the enlargement.
4. Correct exposure during the process of enlargement.
5. The most minute attention to the details of development, including the chemical treatment of the enlarged negative.
The original negative should not be too dense, nor, on the contrary, should it be too thin. If necessary, it should be washed off, or strengthened, as the case may be. Too strong a negative is usually weakened with ammonium persulphate, or the fixing hypo solution is quite sufficient. All spots, points, etc., should be retouched with the pencil and carmine.
The diapositive should be produced by contact in the copying apparatus. A border of black paper should be used to prevent the entry of light from the side.
The correct period of exposure depends upon the thickness of the negative, the source of the light, its distance, etc. Here there is no rule, experience alone must teach.
For developing one should use not too strong a developer. The metol-soda developer is well suited to this work, as it gives especially soft lights and half tones. Avoid too short a development. When the finger laid behind the thickest spot, and held toward the light, can no longer be detected, the negative is dense enough.
The denser negatives should be exposed longer, and the development should be quick, while with thin, light negatives the reverse is true; the exposure should be briefer and the development long, using a strong developer, and if necessary with an addition of potassium bromide.
The silver chloro-bromide diapositive plates, found in the shops, are totally unsuited for enlargements, as they give overdone, hard pictures.
To produce good artistic results in enlarging, the diapositive should be kept soft, even somewhat too thin. It should undergo, also, a thorough retouching. All improvements are easily carried out on the smaller positive or negative pictures. Later on, after the same have been enlarged, corrections are much more difficult and troublesome.