What does reciprocity mean in photography?

Reciprocity means that the image density produced when photosensitive materials are developed is directly proportional to the exposure time and light intensity. (Please read the following in detail _

The function of aperture and shutter is to control luminous flux. The camera lens is marked with the words f2.8, 5.6, 8, 1 1, 16, which is the aperture coefficient. The larger the coefficient, the smaller the aperture; The smaller the coefficient, the larger the aperture. Shutter speed is the abbreviation of fraction in seconds, for example, 125 means1125 seconds. Shutter and aperture are complementary to each other, which is the commonly used reciprocity rate. Reciprocity means that the image density produced when photosensitive materials are developed is directly proportional to the exposure time and light intensity. However, when the light intensity is abnormal and the exposure time is extremely short or too long, this rule will fail, and the result is difficult to predict, so the term "reciprocal failure" has come into being. If you want to keep the exposure constant, the aperture will increase by one level and the speed will increase by one level; If the aperture is reduced by one level, the speed will be slower by one level. For example, if the aperture is 1 1 and the speed is1/60s, when the aperture is changed to 8, the speed will be changed to1125s. When the theme requires a certain depth of field, the aperture size should be given priority. If the background of the subject is messy, choose a large aperture to blur the background; A small aperture is suitable for panoramic shooting, so that there is enough depth of field to show the whole picture. When shooting a moving object, we should first consider the shutter speed, and then match the appropriate aperture.