The use of B-door photography uses B-door to shoot lightning.

Photographers who are good at shooting night scenes point out that one of the best ways to prevent camera shake when using door B at the end of long exposure is to lean against the camera with a piece of black cardboard before releasing the shutter. Another advantage of this method is that you can change the focal length or aperture during exposure. If the camera shakes occasionally during calibration, it is necessary to ensure that the camera has stopped shaking before continuing exposure, which is especially important for shooting with a long focal length lens.

When using B-door for long exposure, the problem that is easy to appear is the so-called "reciprocity law failure" that photographers often say. This usually happens when exposed to color film for one second or more. Fundamentally speaking, long exposure will change the color balance of the film, and this phenomenon is often unpredictable by photographers.

Generally speaking, there is no simple way to solve the problem of the invalidation of reciprocity law, so that every shot can be accurately exposed. David Williams, a British photographer, put forward a simple method, that is, "take more photos with longer exposure time on the basis of the calculated exposure."

Williams pointed out that for long exposure, it is necessary to record the exposure time and aperture used by each photo. In addition, various exposure indexes can be used for shooting to make up for the deficiency caused by the failure of reciprocity law.

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Editor's Note: On Reciprocal Law and Reciprocal Law Failure

Reciprocal law is a photographic term in the film age.

A larger aperture allows more light to reach the film through the lens than a smaller aperture (for example, the aperture of F2.8 is twice that of F4), and a lower shutter speed allows light to stay on the film for a longer time than a higher shutter speed. From this, it can evolve into the following situation: the combination of aperture F2.8 and shutter speed 1/500 seconds, and the combination of aperture F4 and shutter speed 1/250 seconds, the exposure effect is the same. They are consistent with the combination of the middle aperture F8 and the longer exposure time of 1/60s, which is called equal exposure.

All films with equal exposure have the same negative density after development. In the exposure formula, the illuminance and time values can be replaced with each other. For example, the illumination is 100 lux, and the exposure is 1 1 00 seconds, which is equivalent to the illumination is1lux and the exposure is1second. This relationship that illumination and time can be substituted for each other is called reciprocal law in photographic exposure, also known as reciprocal law or reciprocal law.

However, due to the chemical characteristics of thin films, it is difficult for weak light to trigger chemical reactions of thin films. After the illuminance of light is lower than a certain illuminance, even if the exposure time is extended, enough exposure cannot be obtained. Specific to photography practice, that is, when shooting night scenes, if the exposure time of long exposure is calculated according to the formula of reciprocity law, it will be underexposed, which is the failure of reciprocity law.

It should be noted that the photosensitive characteristics of electronic image sensors (CCD and CMOS) are different from those of film, so digital cameras need to reconsider the failure of reciprocity law and cannot simply apply the experience of film cameras. Moreover, image sensors with different production processes have different performances on this issue, and the experience of one camera cannot be applied to another camera.

Since the digital camera can play back the just-shot photos through the LCD immediately after shooting, and check whether the exposure is as expected, even without accurate photometry and without considering the reciprocity law, it can generally get more accurate exposure through simple tests. Therefore, in the era of digital photography, the failure of reciprocity law and reciprocity law can be ignored. However, another problem needs careful consideration-signal-to-noise ratio. When the light is strong, the noise signal of the image sensor (noise, manifested as noise and contrast reduction) is covered by the strong light signal, which is not easy to affect the imaging. But when the light is weak, the problem of signal-to-noise ratio becomes prominent. Moreover, long-term exposure, with the continuous working time, the temperature will gradually increase, and the noise will increase geometrically, which is also the most headache for digital cameras to expose for a long time.

Taking some heat dissipation measures to control the temperature rise of cameras, especially image sensors, is the best way to deal with the problem of signal-to-noise ratio decline.

As far as B-door photography is concerned, one method is to increase the total exposure, and for the exposure of 1-9 seconds, increase the aperture of 1/2l; For the exposure of 10-99 seconds, increase the aperture of 1-2; When the exposure time exceeds 100 second, the aperture increases by 2-3 steps. An indicator like this means that in general, it can't be regarded as a static thing, especially with different films, depending on the specific situation.

In short, to solve the problem of the invalidation of reciprocity law, we mainly rely on experiments, and gradually explore some experiences in practice by repeatedly comparing the recorded contents with the obtained results.