Photographic Effects

various phenomena that disrupt the single-value relationship between the exposure H of a photographic material and the optical density D obtained after exposure of the material. Several dozen photographic effects are known. The following have the greatest theoretical and practical importance.

Solarization is an image reversal observed at large values of H; a second reversal involves the transition to an increase in D with an increase in H at values of H higher than necessary for solarization (see Figure 1 in SENSITOMETRY). Both effects are encountered in practice only at very high overexposures, but they are sometimes deliberately used to obtain specific artistic effects.

The Schwarzschild effect (seeRECIPROCITY LAW FAILURE) strongly influences the results of photographing both very weakly luminous subjects, such as stars, and very strongly luminous subjects, such as explosions.

The intermittency effect refers to the dependence of all parameters of the characteristic curve, including the values of D at a given value H, on whether exposure is the result of continuous illumination or is broken up into n partial exposures H₁, H₂, . . ., H_n (n ≥ 2) separated by intervals of no illumination (provided H₁ + H₂ + … + H_n = H = const). The D depends not only on the division of one total exposure into a number of partial exposures, but also on the method of division—the number and duration of exposures and the frequency of succession. This phenomenon occurs in practice when periodic processes are photographed, such as spark discharge, or when a light flux is attenuated by means of a rotating disk with slits.

Double exposure effects are observed when two exposures by light at different levels of illumination or by different types of radiation produce a total value D that is greater than the sum of D₁ + D₂ in the blackenings from each exposure individually. If the first exposure alone does not produce blackening (D₁ = 0) and its effect only increases sensitivity to the subsequent exposure, the effect is called hypersensitization by means of preliminary exposure. If the second exposure by itself does not produce blackening (D₂ = 0) but only intensifies the effect of the first exposure, the effect is called latent image intensification, or latensification, by exposure to light. Such effects are used in photographing faintly luminous objects.

Temperature effects involve the dependence of D on temperature during exposure at a given value of H, as well as the varying nature of this dependence under various conditions of illumination E. There is a monotonic increase of D with a decrease in temperature at low values of E and with an increase in temperature at high values of E. There is also a complex nonmonotonic change in D with changes in temperature at moderate values of E; the latter effect is typical in most cases of practical photography. Such phenomena can substantially affect photographic results, although they are not always taken into account.

The Herschel effect refers to the partial or complete destruction of a latent photographic image by subsequent exposure with red light or radiation at longer wavelengths. It is an important technique in studying the latent image and the mechanism of its formation.

The latent image may gradually deteriorate during the time between exposure and development. The effect is usually not intentionally promoted and is the result of the thermal or chemical action (or both) of the surrounding medium. As a result of such aging and deterioration, development produces reduced values of D that do not correspond to the actual value of H. Such deterioration affects photographic results if development is postponed for a long time, for example, on expeditions (especially in a hot, humid climate).

The Sabattier effect refers to the complete or partial reversal of an image (a decrease in D with an increase in H at all values or only at small values of H) as a result of uniform exposure of a developed, nonfixed photographic material and subsequent additional development. The phenomenon, which may be deliberately evoked as a means of artistic expression, is an effective technique for isolating regions on the photograph where there are equal values of D.