Each quadrant of the tone reproduction diagram illustrates one part of the photographic process. The true strength of the diagram comes from showing who they interact. As Jones writes about the negative, “it makes little difference what its characteristics are, provided a print of satisfactory quality can be made therefrom.” For example, if the goal is to have maximum separation of the shadows, the gains from picking a film that emphasizes the shadow contrast can be lost with the wrong choice of paper. This can only be known by evaluating the process as a whole.
Perhaps the most important quadrant is the tone reproduction curve. It is the summation of all the steps in the process. With proper interpretation, the curve can be used to evaluate how a change in one part of the process effects the whole. For instance, it will show how the expansion of the shadow contrast will cause a contraction in highlight contrast. Yet even the tone reproduction diagram doesn’t tell the whole story. As it is only concerned with the objective reproduction of tones. It doesn’t explain how the tones in the photograph will appear to the viewer.
One test evaluated how tones are perceive through the effects of adaptation and simultaneous contrast under different levels of illumination. The test used a gray scale containing five steps of known luminances. It found that “decreasing the illuminance by a factor of ten caused the brightness of the white step to decrease by a factor of less than two times. A dark gray step, however, remained the same in brightness. It showed perfect brightness constancy. The darkest step on the scale actually increased slightly in brightness when the illuminance decreased. The remarkable finding is that a ‘black’ object of low reflectance becomes brighter when the illuminance is decreased and darker when the illuminance is increased, whereas a ‘white object’ shows the opposite trend.” So how the tonal relationship of a photograph can change depending on how it's illuminated. The way I interpret this is shadow contrast appears greater under higher levels of illumination.
The following graph shows the effects of light and dark surround on the perception of brightness in a photograph. Not only would this apply to what surrounds the entire photograph, like a matte or wall, but how the tones respond to other tones within the photograph itself as the second set of examples illustrates.
This last example illustrates local inhibition and adaptation of the eye on a gray scale. The x-axis is luminance and the y-axis is brightness. The first graph is of the gray scale surrounded by a medium gray background. The second example is of the gray scale surrounded with a white background. The white surround makes the gray scale appear darker, increases the apparent contrast of the three lightest steps and decreases the contrast of the two darkest steps. The third has the gray scale with a white background as well as white separating each step which shows an even greater effect than the second example.
The distribution of the tones in the photograph appears to play a part in the perception of tonal relationships that is separate from the one the negative and print contrast plays. Something to keep in mind the next time someone attempts to judge photographic quality based only on the negative curve.