What is density?

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Density refers to the number of silver particles deposited per unit area of negative film after exposure, development and fixation of photosensitive materials, which is used to indicate the degree of blackening. This black color is usually called density.

In the process of exposure and development, the silver halide in the emulsion layer of photographic film is reduced to different amounts of metallic silver according to the brightness of the photographed scene. Metallic silver is an opaque granular substance, which can absorb and block light. Explained from the brightness of the scene, when the shot scene is illuminated by strong light, its backlight intensity is stronger. After photographic exposure and development, the brighter the part, the denser the reduced silver particles, the more silver particles deposited and the lower the transparency. It is observed that there is more light blocked by shading and less light transmitted. On the contrary, the darker the part, the more dispersed the reduced silver particles are, the less silver particles are deposited, and the higher the transparency is. It is observed that less light is blocked by the shade and more light is transmitted.

Different amounts of silver particles deposited on the negative film constitute images with different blackness. Because density has different degrees of shading ability, it can be determined according to the amount of transmitted light.

In addition, the relationship between density and image effect is also provided.

From the perspective of photographic sunlight, the greater the exposure, the overexposure of the film, the more silver particles deposited, the deeper the blackness of the image and the greater the density of the film. On the contrary, the smaller the exposure, the lower the sensitivity of the film, the less silver particles are deposited, the lighter the blackness of the image and the smaller the density of the negative film. It can be seen that the exposure directly affects the density of the film. Neither overexposure nor underexposure can obtain the "ideal density", which can only make it lose the level of light or dark.

The perfect image effect depends on the negative density produced by correct exposure. Any overexposed or underexposed film is difficult to print photos with ideal image effect. The overexposed negative has a deeper blackness and higher density in the bright part of the scene, and the printed photos and images can only show some shadow lines and layers in the shadow part of the scene to a limited extent, while the shadow lines in the bright part are white without hierarchical texture. Under-exposed film, the brightness of the scene is greatly lost, and the film is generally light in blackness, low in density and transparent or translucent. Printed photos and images can only show some light and dark levels in the bright part of the scene, while the light and dark levels in the shadow part disappear into gray-black tones. It goes without saying that if you want to get the image effect with rich layers, normal contrast and bright tone, you can only rely on the negative with correct exposure and normal density.