Contrast and Density

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The basic, perceptible properties of film are density and contrast. Contrast is the tonal difference between the highlights and shadow areas of the subject, Density is the difference between the amount of light striking the film and the amount light that actually passes through the film. In other words, density is a measurement of how much transmitted light passes through the film. The contrast of the film is dependent on the amount of exposure on the shadows as well as the amount of development of the film. Contrast is the tonal ratio of shadows to highlights. The differences in the density of the film determine its contrast. Density is technically defined as the ratio between the light striking the film and the amount of light passing through it. Density, then, is really a measurement of the amount of transmitted light in a film. An example of a contrast and density curve can be seen in Figure 3.16.

If you use black charcoal to draw, the harder you press on the paper, the darker your drawing gets. If you have very white paper, the black charcoal together with the white paper gives you a certain amount of contrast. But what if you use a number-two pencil instead of charcoal to draw? Will you get the kind of blackness with the number-two pencil that you achieved with the charcoal, even if you press very hard? Of course you won't. Why? Because the charcoal and the pencil each have their own built-in contrast capability that cannot be changed. It does not matter whether you use different papers for this test; the charcoal ren-


Figure Showing density and contrast in film.

Figure A charcoal drawing versus a pencil drawing.

In photography, you control the contrast in two ways: by exposure and by development. The more exposure you give a film, the higher contrast it has, and vice versa. However, exposure alone does not determine the amount of contrast. The development process also affects contrast. The exposure determines the amount of silver halide crystals that are converted into metallic silver, and the development process determines how much conversion happens.

Remember that the shadow areas of your subject reflect less light on the film than do the lighted areas of your subject, so the shadow areas and the middle tones affect fewer silver halide crystals. The highlights of the subject, however, reflect a great deal of light back to the film. Since photochemistry reacts with the exposed areas of the film, more metallic silver will be converted in the highlights than in the middle tones and shadow areas. The developer both physically transforms the silver halide and chemically changes it. But the developer does need time to penetrate all the areas of the film. The development penetration process depends on the time and temperature of the developer. In general, temperature speeds up any chemical reaction as well as makes the gelatin more "open" to reaction due to expansion. This photochemical penetration creates density. Penetration determines how much physical and chemical change takes place in the exposed silver halides.

As we've seen, density in photography is the ratio between the light striking the film and the amount of light actually passing through it. In short, density is a measurement of transmitted light. As discussed, density in film is dependent on the initial exposure and the amount of development. It is the amount of density in a film that creates the perception of contrast. If you can readily see through a negative after development, the film is said to be thin or of low density. If, however, it is hard to look through the negative, the film is said to be dense. Refer to Figure 3.18 for an illustration of density in film.

Thin negatives can be intensified and dense negatives can be bleached, or lessened in intensity. Intensification is done through the use of another metal, such as selenium, to replace and "intensify" the metallic silver. Bleaching is done with potassium ferrocyanide. Intensification and bleaching are still practiced by still photographers. However, it is always better to control density through exposure and development rather than through a postprocessing

Processed films that have a good balance of highlights, middle tones, and shadow areas are called normal. This does not mean, however, that there is a standard of "normal" negatives by which others are judged. Normal merely refers to the good separation of highlights, middle tones, and shadows as captured on film. A particular frame could be predominantly black but still have a good tonal range; the same could be true of a snow scene, which is mainly highlights of white. These scenes are "normal" if there are good gradations from

Remember the compression and shifting of the tones in the H and D film characteristic curve? If the hardware used in photography is reliable in the way it exposes each frame, and

Figure Density in films.

if the processing procedure is controlled and the exposure of each frame is consistent, it is possible to have the ultimate control of the tones present in the negative. This kind of control is aimed to do only one thing: to steer the photographic process to obtain a specific print. This kind of control enables the photographer to place the important tones where it is possible to "slide" the tonality in the direction desired.

With the use of filtration, reliable equipment, processing procedure calibration, and visualization, photographers Ansel Adams and Fred Archer were able to devise such a process, called the zone system. Understanding this system is critical to understanding tonal relationships in 3D lighting.

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