When printing positives either on paper or on plates for lantern slides, working conditions are somewhat different, no camera being used and the object reproduced being a negative instead of the original subject. The range of contrast in negatives is frequently much greater than in natural objects, but the exposure is governed by the same conditions as those which apply in the negative making. Such an exposure must be given that the greatest opacity which it is desired to print through at all just produces a visible deposit. Usually the highest light of all should be printed free from any deposit, and the next tone to this should be taken as the one to be printed through.
Turning to the curve shown in Fig. 53 and considering a bromide paper, this will have a speed of about 5 on the speed scale, so that it is 50 times slower than the film which we considered first and the point marked 1 on the exposure axis corresponds to an exposure of a candle-foot-second. Now the highlight which we shall want to print through in an average negative will let through only about l/20th of the light, so that we shall have to give such a bromide paper an exposure of 20 candle-foot-seconds behind such a negative, and for a paper or plate of any speed we may write P where P is the speed of the paper and 0 is the opacity of the highlight in the negative which it is desired to print through. If the highlight in a negative lets through l/20th of the light, then the opacity of that negative is said to be 20. If it lets through l/100th, it is 100, and so on.
E = -----------