The law of reciprocity refers to

The law of reciprocity, a term from the film photography era. Also known as "reciprocity law" and so on. It means that aperture and shutter speed can be directly proportional to each other while the exposure remains unchanged.

Shutter speed greater than 1 second or less than 1/1000 second will cause the reciprocity law to fail. There is also a situation called reciprocity law failure: as you know, the way to control the exposure can be to change the shutter speed or aperture. You also know that if you change two factors at the same time, the exposure will be equal. For example, the following exposure control indexes: f/2, 1/1000f/2.8, 1/500f/4, 1/250f/5.61/125.

In the above two examples, if we reduce the aperture by one stop and increase the exposure time by one stop, what will be the result? The amount of light hitting the film is still the same. This relationship between aperture and shutter speed is called a reciprocal relationship. A change on one side is equivalent to a change on the other side, resulting in the same exposure.

For film, regardless of whether you use f2 at 1/1000 second speed or f5.6 at 1/125 second speed, the amount of light hitting the film is the same under the two exposure methods. This is a common example of reciprocal exposure. But under certain extreme environmental conditions, this reciprocity relationship is destroyed, which we call the law of reciprocity failure.

The reciprocity rate fails

When will the reciprocity relationship be destroyed? That is when the exposure time is extremely short, such as 1/50000: or when the exposure time is very long, such as 10 minutes, you may say: Such an exposure time is unrealistic. Who would expose film at 1/50000 of a second or 10 minutes? The answer is that exposure like this is much more realistic than you think.

Let’s talk about short exposure first. When you use a strobe to expose, it's actually possible to know how long the strobe's one flash lasts. The problem occurs with short exposures like this, when the light hits the film for such a short time that the silver halide crystals in the emulsion don't get enough time to cause them to react. It seems that silver halide crystals also have a little bit of inertia, and they are not flammable.