This section is from the book "The Fundamentals of Photography", by C. E. K. Mees,. Also available from Amazon: The Fundamentals Of Photography.
In order to find out how closely the tones of the print follow those of the original subject we must follow the changes of these tones through both steps: we must study first how far the negative reproduces in an inverted form the tones of the subject and then how accurately the printing paper inverts these again to give a representation of the original.
Any silver deposit in the negative will let through a certain proportion of the light which falls upon it. A very light deposit may let through half the light, a dense deposit one-tenth, a very dense deposit one-hundredth or even only one- thousandth. The amount of deposit through which one can see depends, of course, upon the brightness of the scene at which one is looking, but it is interesting to note that one can see the sun through a deposit which lets through only about one-twenty-billionth of its light.
These fractions of the light which are let through are referred to as the "transparency" of the deposit, and the inverse of the transparency is called the "opacity", the opacity, therefore, being the light-stopping power of the deposit. A deposit which lets through half the light, for instance, is said to have a transparency of 1/2 and an opacity of 2. Similarly, one which lets through one-tenthof thelight has a transparency of 1/10 and an opacity of 10.
If the negative is to be the exact inverse of the scale of tones of the subject, then the opacities of the different areas must be in proportion to the brightnesses of the parts of the subject which produce them. In Fig. 67 we have a subject in which if we take the black background as having a brightness of 1, the brightest portion will have a brightness of 10, and the other portion will be in proportion. Then when we make a negative of this we shall get the picture shown in Fig. 68, and in this, if we measure the opacities of the negative, we ought to find them exactly inverse to those of Fig. 67, so that the transparency of the background, A, would be ten times that of the table, B, or the opacity of the table, B, will be ten times that of the background, A. Not only this, but the relative opacity of the deposits in the areas C, D and E should also be the same as the brightnesses of C, D and E in the original subject.
Fig. 67. Five Toned Block.
Fig. 68. Negative of Five Toned Block.
Fig. 69. Graded Strip of Exposures.
It will be seen by the foregoing, therefore, that a technically perfect negative will be one in which the opacities of its different gradations are exactly proportional to the light reflected by those portions of the original subject which they represent.
Let us now consider how far we can fulfill this condition and what must be done to obtain such a perfect negative of any subject.
Suppose that a photographic plate or film is exposed to a series of known brightnessess; for instance, that we photograph a scale made up of steps of different reflecting powers so the brightness of each step is doubled with regard to the next one.
The result that such a series of exposures will give has already been discussed in Chapter VI (Exposure), but we must now look into the matter somewhat more carefully. We shall get a negative which will look like Fig. 69. Now if the rendering is technically perfect, the opacities of this negative should be the same as the brightnesses of the different steps of the original; that is to say, as each step is twice the brightness of the next step, the light let through each step of the negative should be half the amount of the step next to it.
This would be attained if each step in the negative added the same amount of silver to the deposit, so that if we could represent the silver for each step as altering the thickness of the silver deposit (it does not do this really, of course; it adds to the number of grains in the same layer) and then could cut an imaginary section through the negative so as to show the height of the deposit of silver, it should look like Fig. 70; and if we draw a diagram in which the amount of silver is represented by the height of a vertical line, the diagram showing the amount of silver for the different steps might look like Fig. 71.
Heights of Silver Deposits (diagram).
Heights of Silver Deposits. (Line Diagram).
If we actually try this experiment, however, we shall find that the silver does not rise quite uniformly in this way as the exposure is increased through the entire scale, but that instead we get the diagram shown in Fig. 72, and this diagram, which represents the actual relation between the silver deposit in a photographic material and the increase of exposure, requires careful study.
Starting at A and proceeding to B we notice that at the beginning, in the lower exposures, the steps are marked by a gradually increasing rise, and, therefore, in this part of the exposure scale there will be too great a gain in opacity for each given increase of exposure. A negative, the gradations of which fall in this period, will yield prints in which an increasing contrast is shown between tones of uniform increase of brightness; that is to say, it will appear what we term "under-exposed." From this period at B we pass imperceptibly into the period where the densities show an equal rise for each equal increase of exposure, and here we have our technically perfect negative, that is, one in which the opacities are exactly proportional to the light intensities of the subject. This is termed the "period of correct exposure," and only through this period of the curve where the opacities are directly proportional to the exposures and where the densities show an equal increase each time the exposure is doubled shall we get a perfect rendering of the original subject. From the point C onwards we have a gradually decreasing rise in the steps with increase of exposure until, finally, the increase of density with further exposure becomes imperceptible. This period is the period of "over-exposure," in which the opacities of the negative fail to respond to increasing amounts of exposure and the correctness of rendering is again lost. It will be seen at once, then, from this curve that only through the period of correct exposure where equal increases of exposure are represented by equal rises in density can tones of the original subject be correctly reproduced in the print.