What is the specific explanation for the failure of the law of reciprocity?

We have discussed the various properties of black and white and color films, now let us turn to a different topic, called reciprocity failure (also translated as reciprocity failure). As you know, you can control exposure by changing 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 index:

f/2, 1/1000 f/2.8, 1/500

f/4, 1/250 f/5.6 1/250 < /p>

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 both 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.

When does the reciprocal relationship break down? 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. Now, you may say: "Such an exposure time is unrealistic. Who would use 1/ What about exposing film for 50,000 seconds or 10 minutes? "The answer is, exposures like that are much more realistic than you think.

Let’s talk about short exposure first. When you use a strobe light to expose, how long is the flash time of the strobe light? 1/1000th of a second? 1/10000th of a second? Or 1/50000 second? These are actually possible. 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 inertness, and they are not flammable. It also takes a little time to get them to act in unison. Therefore, exposure times that are too short have no time to overcome their inertia. The result is underexposure.

The other extreme: what about very long exposures? Any photographer who shoots in extremely dim light needs long exposures. For example, in astrophotography, exposures of 10 minutes, 20 minutes, or even several hours are not uncommon. To photograph a dark scene, such as a stable under moonlight, you may want to expose your subject for 30 seconds. Unfortunately, silver halide crystals begin to lose sensitivity when exposed to light for too long. For example, a 20-minute exposure does not produce twice the effect of a 10-minute exposure; even a 30-second exposure does not produce twice the effect of a 15-second exposure.

So, this is called the failure of the law of reciprocity. This is a problem that occurs with both black and white films and color films. In fact, this problem is more serious for color films, because it not only affects the "density" of the image - if you ask negative film to take this risk, the resulting image will be too thin - but also affects the color of the image. It should be considered that if you use color film and the exposure time is less than 1/1000 of a second or greater than 1 second, reciprocity failure may occur.

How to solve the problem of reciprocity law failure of color film?

First of all, you can set the exposure time in a range where you don't need to worry about reciprocity failure. For example, if you originally wanted to shoot with exposure conditions of 1/2000 and f/8, you can change the exposure index to increase the trigger time of light on the film. If the shutter speed is changed to 1/500 and the aperture is f/16, the net exposure value is the same, but the time for light to trigger the film is within a safe range.

If you can't do this, there is still a second way. Kodak has two types of film produced to avoid the problem of reciprocity law failure: "Wanli III Professional S-Type Color Film", which is designed for extremely short exposure times. Exposure time can be from 1/10 to 1/10000 seconds. No need to add color filters or other adjustments to exposure conditions. This film is generally called "VPS" and is available in loose sheets and rolls in all major sizes. It has been mentioned that VPS is a type of film that many portrait photography experts are happy to use. Because it expresses the human skin color extremely vividly.

"Wanli II patented L-type professional color film is designed for extremely long exposure times---1/50 seconds to 60 seconds. This film is only available in two packaging sizes: loose leaf and 120 film. There is no 35mm specification.

Finally, you must know that if you cannot use the above two types of film and cannot shoot within the safe range of the exposure index, you will have to face the problem of reciprocal failure, but not All efforts are in vain. The instruction manuals of different film manufacturers have information on how to compensate for the failure of the reciprocity law during exposure and processing. You can refer to it for review.