What is the reciprocity rate in photography?

Reciprocity law failure (also known as reciprocity law failure, reciprocity rate effectiveness, etc.). )

After the film is exposed, it will undergo chemical changes, and after development, an image will be formed. This change degree of emulsion is measured by exposure. Exposure is the product of illumination and time on the image surface. As long as the total exposure is constant, the aperture value and shutter speed can be adjusted with each other. For example, the same EV value can be combined with different shutter speeds and aperture values, and the exposure time is doubled, which requires the aperture to be opened one step; The exposure time is doubled and the aperture is smaller. This is a reciprocal relationship.

This relationship is generally applicable, but when it takes a long time to expose under extremely low illumination or the flashing time of electronic flash is very short, the reciprocity law will fail. For black-and-white films, the reciprocity law will fail when the exposure time is shorter than11000 seconds or longer than 1/2 seconds. In both cases, the emulsion no longer guarantees its normal sensitivity, resulting in underexposure. The failure index of color film reciprocity law is slightly wider under long-term exposure, which is generally bounded by 1 second. When the reciprocal law fails, not only the density, sensitivity, contrast, color sensitivity and tolerance will change, but also the color balance will change, so it is necessary to use filters to compensate. For professional filters, because of the different failure conditions of reciprocal law of three-layer emulsion, two kinds of compensation filters, long exposure type and short exposure type, are specially made.

In specific shooting, we should pay special attention to the exposure time is too long or too short, as far as possible to avoid. For example, I originally planned to shoot a fast-moving object with f8,11000 seconds, but I was worried that the image definition might not be high, so I changed it to f5.6, 1/2000 seconds. Although the total exposure is constant in calculation, the same exposure effect should be obtained, but the reciprocal law fails, resulting in underexposure. At this time, if the moving direction is slightly changed, the exposure will be improved by shooting with f 1 1 and 1/500 seconds. Similarly, the appropriate exposure is f 16 and 1/2 seconds, but in order to control the effective depth of field, f22 and 1 second are used instead, resulting in underexposed film.

When the exposure deviates from the normal exposure time, the failure of reciprocity law becomes more obvious. If 1 sec and 1/2000 sec are slow exposure and high exposure respectively, the failure of the reciprocal law is minimal, and then the effect will be obvious at1sec and 1/5000 sec. The fault is extremely obvious at 100 second and110000 second. When using electronic flash, we should pay special attention to the failure of reciprocity law caused by the ultra-short flash time of the latter.

The failure of reciprocity law caused by too long or too short exposure time is rarely encountered in advertising photography in the studio at present. Large professional electronic flash has high power, the standard flash time is not too short, and it can also be used in combination with multiple lamps. The duration of ultra-high-speed electronic flash will only appear when the small automatic flash is at a short distance. In other indoor and outdoor low-light conditions outside the studio, such as shooting at night or in a bar, the exposure time is often too long. When advertising trick photography, there are often short exposures.

If the objective conditions can't ensure that the exposure time is within the normal range, we can only compensate for the underexposure by compensating the exposure time and using the compensation filter, and even adjust the development time to compensate for the contrast change.