As it has been shown that with a given negative there is one kind of paper which (say, for contact printing) will give the best results with full development, it is easy to see that the exposure must be such that the most dense part of the negative shall be represented in the finished print by white paper or a faint tint.
It is here that the advantage comes in of not allowing any variation in the time of development, but always carrying it to the full, because it is vastly more easy to arrive at the correct result when only one condition of working (exposure) is varied at once.
Make this, then, the starting point in every trial - Find the exposure that will just give upon full development the faintest detail desired in the most dense part of the negative. The manner in which the shadows are then found to have been rendered will at once show how far the paper chosen has been correct or otherwise.
If the shadows are not deep enough the paper is too rapid, because, generally speaking, the more rapid the paper the longer the gradation. If there is not all the detail shown in the shadows which it is thought the negative should render, then the paper used has been too slow. If, however, the most rapid paper has already been used in making the trial, then the special treatment to be described later will probably prove useful.
Naturally one would not make use of any extra treatment unless compelled to do so, both on account of the extra trouble, small as it may be, and also because the result probably would not be quite equal to that which could be obtained from the paper which was in itself most suited to the negative.
As it is very desirable to be able to record exposures so that they can readily be repeated at any time, a systematic method should be arranged from the outset. The following will be found useful for all varieties of contact printing.
Source of Light.
Bromide Papers. Lamp or gas (No. 5 Bray Flat Flame).
Gaslight Papers. Gas (No. 5 Bray or incandescent), or magnesium ribbon.
Some plan should be arranged for making exposures at definite distances from the source of light, so that the correct amount may be given in, say, 10 or 20 seconds. A useful scale may be arranged by marking off distances as shown below upon a board about 4 feet in length.
If the source of light is placed at A, and the distance of one metre or one yard is taken as the unit, then the value of the light will be as shown by the numbers, if the distances are marked off thus :
Distances in Inches.
Metre . .
Yard . .
If it were then found that a certain negative and paper required 20 seconds' exposure to the Bray gas, when placed at 1 (metre), the needed information could be registered as 20 G.M.S. (20, gas, metre, seconds). When using a slow bromide paper, if it were found that 120 seconds should be given at point 1, then the entry would be made 120 G.M.S., though to save time in printing the actual exposure might be 15 sec. at point 8 giving the same result. In the same way when using a gaslight paper, M. (minutes) would have to be substituted for seconds, or the required length of magnesium ribbon and distance noted. (One inch of magnesium ribbon 1/8-in. wide is about equal to a No. 5 Bray burner at one metre, burning for 10 minutes).