Photography practice in which reciprocity law is invalid

In photography practice, the light sensitivity of the photosensitive film is generally limited to a certain range to produce normal exposure effect. If it exceeds the specified range, it is impossible for the latex layer to strictly abide by the exposure law. When the illumination of the photosensitive layer of the photosensitive film is too large or too small, or the exposure time is too fast or too slow, the exposure with the same amount may produce different exposure effects. For example, when the light is weakened to a very low degree, even if the exposure time is relatively increased, the satisfactory exposure effect may not be obtained; On the contrary, when the light intensity is extremely bright, shortening the exposure time to a very short moment with a very high shutter speed may not necessarily achieve a good exposure effect. The above two phenomena are called light exhaustion. For example, when shooting the starry sky with weak brightness in celestial photography, or shooting with electronic flash in a very short moment, there will be the phenomenon of light exhaustion. Different photosensitive materials have different degrees of light attenuation, and color film is more prone to light attenuation than black and white film. This phenomenon is caused by the reaction inertia of photosensitive film to light. The light is very dark or the exposure time is very short, so it is difficult for the light to penetrate the inner layer of emulsion, which reduces the photochemical effect of the film. Assuming the same film is used, the illumination is 1000 lux, and the exposure is 1 sec, which is higher than the silver shadow density generated by the illumination is 1 lux and the exposure is 1000 sec. This phenomenon is the failure of the law of reciprocity.

In the process of photography, in the shooting stage, in the darkroom amplification stage, in black-and-white film and color film photography, the failure of reciprocity law is often encountered. At dusk and dawn, and in the case of dark indoor light, the exposure time is very long, which often leads to underexposure or color cast.

The failure of reciprocity law should be grasped in photographic exposure. When using all kinds of films, we should fully understand their performance, especially pay attention to the reciprocal law exposure increment data provided by film manufacturers. For example, Kodak 35 mm film in the United States was shot in dim light. When the exposure time measured by the exposure meter is 1 sec,1sec and 100 sec respectively, the actual exposure time should be 2 sec, 50 sec and 1200 sec respectively. On the contrary, when shooting under high-speed electronic flash, if the exposure time measured by the exposure meter is110000 seconds, it is necessary to increase the aperture by half a step compared with the normal exposure.