What is the principle of reciprocity rate?

The reciprocity rate can be replaced with illumination and time in the exposure formula. The relationship is E=I·T (E is the exposure, I is the illumination, and T is the exposure time). This relationship is In photography, it is called reciprocity rate (reciprocity rate, reciprocity rate). In the film era, each type of film had its optimal exposure range. For general daylight film, the optimal exposure range is usually between 1 second and 1/1000 second. Within this range, to maintain the same exposure and open the aperture one step wider, the shutter speed must be adjusted one step faster. The exposure of 1/250, f/4 is equivalent to the exposure of 1/500, f2.8. Reciprocity failure: Because the photosensitive emulsion is inert to light, it is difficult for light to penetrate when the light is extremely dark (for astronomical photography to capture weak starlight) or when the exposure time is extremely short (for photography in a very short time using an electronic flash) The inner layer of photosensitive emulsion will reduce the photochemical effect of the film. If the same exposure is applied to the same photosensitive film, different densities will be produced. This phenomenon is called reciprocity failure in photographic exposure. When using film, pay attention to the data on reciprocity characteristics provided by the manufacturer.