502. The Gradation of a Sensitive Paper. Let us suppose that a certain sensitive paper, on free exposure to a source of light for one minute, becomes tinted just sufficiently to allow of its being distinguished from the original white, and that after 30 minutes' exposure it attains its maximum density, which we will call black.
If this paper is used for printing from a negative of which the highest and lowest opacities are in the same ratio as the exposures required to give the darkest and lightest tones on the paper, that is, i and -Ath, a complete picture with tones ranging from pure white to full black will be obtained on exposing to light for a period of time slightly less than 30 times that required to give a faint tint under the lightest part of the negative.
In this way, none of the tone values of the negative are lost. The paper receives enough light to produce a black under the lightest parts of the negative, while the quantity of light transmitted by the densest parts is not sufficient to produce the slightest tint, but the paper is just tinted under the slightly less dense portions. Thus, any differences in density are faithfully reproduced in the same ratio on the positive print.' If we wished to use the same sensitive paper with a negative the highest and lowest opacities of which are only in the ratio of 20 to I, the positive print would not be able to give black and white simultaneously, while the steps between the successive tones would be consider ably reduced. Instead of ranging from white to full black, all the tones would be comprised between white and a certain depth of grey, or between another grey and black. Actually, full black could only be obtained under the lightest parts of the negative after 30 times the exposure necessary to produce the first perceptible tint had been given, but with only 20 times this exposure the paper would have begun to darken under the densest parts of the negative.
Lastly, if we were to use a negative having highest and lowest opacities in the ratio of 40 to 1, with the same paper, we would obtain all the tones ranging from white to black, but a certain number of tonal values of the negative would be lost either in the light or the dark tones or at both ends of the scale at the same time.
Actually, with an exposure giving a faint image under the densest parts of the negative, that is to say, an exposure 40 times that which slightly tinted the paper under the least dense portions, the paper would be completely black ened under all those portions of the negative of which the opacities are between iand flrths. If, in order to preserve the darker tones, the exposure had been stopped at 30 times that necessary to tint the paper under the lightest part, there would be no image under those parts of the negative where the opacities range between and *it).. Thus a certain number of the light tones would merge into the white of the paper.
All that has just been said for print-out papers applies equally to other papers, i.e. those giving a silver image by development or a pigment image of any kind.
503. The ratio of the quantities of light which respectively give the darkest tone and the lightest tone which can be distinguished from white, for a given photographic paper, is often called the total gradation or gradation of the paper." We have just seen that to obtain the best rendering from a paper, the prints must be made from a negative of which the contrast (the ratio of the highest and lowest trans parencies or opacities of the negative) is equal to or less than 2 the gradation of the paper in question.
The table given below is a rough indication of the average gradations of various types of sensitive paper. It also indicates the range of extreme transparencies of negatives usually termed weak, vigorous, etc.
Fortunately, a certain amount of latitude exists. Thus, a dark grey often appears black by contrast with a light tone, while on the may be done either in a printing frame or in a printing machine or box, the latter being a more or less complicated form of printing frame com bined with a box containing a source of light.' These pieces of apparatus are essentially intended to ensure as perfect a contact as possible between the negative and the sensi tive positive paper throughout the printing, other hand, the sacrifice of a few details in the shadows of the picture does not always spoil the effect.