Application of the principle of reciprocity

When setting the aperture and shutter, we generally follow the "law of reciprocity", also known as the "law of reciprocity".

The so-called reciprocity law refers to the technical means to achieve accurate exposure in order to obtain different depth of field effects or dynamic feelings, and the reciprocity of aperture coefficient and speed level under the same lighting conditions. For example, the accurate exposure for shooting an object is F8,1125s. For the effect of depth of field, if the aperture coefficient is adjusted to F5.6 (increased by one step), the fast speed will be reduced by one step and adjusted to 1/250 s ... For the sense of motion, if the aperture coefficient is adjusted to F 16 (decreased by two steps), the shutter speed will be correspondingly increased by two steps and adjusted to 1/30s. (F8,1125s), (F5.6, 1/250s), (F 16, 1/30s), the above three pairs of different aperture and speed combinations are exposed in the same way, and the subject is photographed.

Color balance

The characteristics of color film are quite different from those of black and white film. When shooting with black and white film, whether the light source is sunlight, flash, floodlight or household light bulb, you can use the same film. But you can't do this with color film.

What is the reason? The essence of light. As you know, the so-called "white light" is actually a mixture of different colors of light. The true nature of this mixed light depends on the color temperature of the incandescent light source. For example, Tai * * * is extremely hot and produces very blue light; Candles are low in temperature and emit red and yellow light. Generally speaking, the higher the temperature, the bluer the light produced; The lower the temperature, the more yellow, orange and red light is produced. Please refer to Figure 4.34. You can see the color temperatures of different common light sources. Color temperature is measured in Kelvin, which means that the temperature is higher than absolute zero. The unit of measurement is "K".

The ideal color film should be that the color of the shot image can be accurately restored to the natural color of the original scene regardless of the light source. But I can't. Especially the color reversal film, it can only get true color reproduction when the color temperature range of the light source is relatively narrow. Therefore, two basic types of color reversal films are produced:

1. Sunglow film is suitable for light source with color temperature of 5500K K, so the color can be accurately restored only when the light source is blue. It needs to use sunlight or flash lamp, and the color temperature is close to 5500K, in order to obtain accurate color reproduction.

2. Lighting (incandescent lamp) color card can accurately restore the color under the light source with red component. So you need to shoot under floodlights or household lights.

As can be seen from the above table, there are two kinds of overflow bulbs.

One color temperature is 3200K and the other is 3400K. Most tungsten lamp films are called B-type films, and the suitable color temperature is 3200 K. A few lighting films designed to adapt to 3400 K are called A-type films. In this way, you can choose the color film that is closest to the color temperature of the light source you use.

If you shoot scenes under ordinary household light bulbs indoors, the color temperature of such bulbs is about 2800K K, so you'd better choose B with a color balance of 3200K, so as to achieve a closer color balance. If you need extremely high shooting accuracy, you must add a filter to the camera lens. We will discuss it in the lesson "Filters".

Color reversal films are usually divided into sunshine films and lighting films, while color negatives are indistinguishable. Most of color negative is just sunlight. If you want to shoot under an incandescent lamp, you can add a filter to adapt. If the formed color edge is deviated, it can be adjusted when printing photos. However, if you use color reversal film to make transparent positive film, there will be no such adjustment opportunity. The developed film is its final product. Therefore, when using color reversal film to photograph, we must pay great attention to its adaptability to the color temperature of light source.

Color negative film has a wide range of color adjustment. Kodak color 400 produced by Kodak can be equally suitable for lighting and sunlight without adding filters. However, please note that Kodak sometimes attaches a notice: "In strict applications, if necessary, appropriate color compensation filters can be added." In other words, when you use Kodak Color 400 to shoot under a tungsten lamp, you need to use a filter to get the most accurate color possible.

To sum up:

Color negative is basically in balance with the sunshine, and it is designed to adapt to the sunshine or strongpoint.

Color retroreflectors should be selected according to different light sources:

A. solar film adapts to sunlight and frequency flash.

B-B tungsten lamp can adapt to 3200K and can be used for household light bulb lighting.

C. Lighting of tungsten lamp A-type film 3400K photographic floodlight.

The failure of reciprocity law

We have discussed the characteristics of black-and-white film and color film. Now let's turn to a different topic, called reciprocity rate invalidation. As you know, the way to control exposure can be to change the shutter speed or aperture. As you know, if you change two factors at the same time, the exposure is equal. For example, the following exposure control indicators:

1/ 1000

f/4， 1/250

In the above two cases, what happens if we reduce the aperture by one step and increase the exposure time by one step? The amount of light emitted to the film is still the same. This relationship between aperture and shutter speed is called reciprocity. The change of one side is equivalent to the change of the other side, and the exposure is equal. Whether you use f2 with a speed of11000 second or f5.6 with a speed of1125 second, the amount of light irradiated on the film is the same under the two exposure modes.

This is a common example of mutual exposure.

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