How to control exposure

Light is the foundation of photography art creation, and photography depends on light.

What does light have to do with photography?

From a technical point of view, the reason why various scenes can be changed from visible images in the objective environment to visible images in photos is due to the optical imaging of the lens and the photochemical reaction of the photosensitive film through exposure. Exposure is the key to determine the quality of photography.

From the artistic point of view, light is an indispensable condition for on-site photography and painting, and it is also an important means to shape the subject image. The use of light is the key to determine the quality of photography.

Photography relies on exposure technology, that is, by controlling exposure correctly and skillfully.

① the meaning of exposure

The camera in our hands is facing nature, which is a colorful and multi-level world with endless light changes. The purpose of photography is to record all kinds of things in real life with photosensitive film. It all depends on the correct exposure of photographic film. There is no doubt that it is impossible for a negative with incorrect exposure to obtain a good artistic effect.

Beginners, the first problems they come into contact with are probably related to photographic exposure, that is, they want to get an ideal photographic work through correct exposure. In order to solve this problem, it is necessary to study and master the theory and concept of exposure control. ● Definition of exposure

Exposure is to let the light pass through the lens to form image light, enter the black box and reach the photographic film, so that the photographic emulsion of the film produces latent image under the photochemical action.

In the emulsion layer of photosensitive film, tiny silver halide crystals with photosensitive function undergo potential photochemical changes under the irradiation of light, forming images that cannot be observed by human naked eyes, so it is called latent image. After the exposed film is developed and fixed, the latent image becomes a visible image, and then the image is a negative image. After the negative film is developed and enlarged in a darkroom, it becomes a positive film with the same brightness as the original scene, which is the final product of photography-an ideal photo. In the whole process of photography, the primary factor affecting the formation of negative image density on negative film is the determination of exposure combination, followed by film processing technology. These two factors are the key to determine whether the film density is correct or not.

In short, the meaning of correct exposure should be: at the right time, expose the photosensitive film with the right amount of light.

The definition of exposure, if scientifically explained, is: the intensity of light multiplied by the time of light action. The "light intensity" in the definition refers to the intensity of light shining on the photosensitive film, that is, illuminance (I stands for illuminance, in lux). The "exposure time" in the definition refers to the exposure time (t stands for exposure time, in seconds) when the photosensitive film is irradiated by Love rays. Exposure is measured in lux seconds. Take e as the exposure, and the exposure formula is as follows:

E (exposure) = I (illuminance) ×T (exposure time)

According to this formula, if you want to get a certain exposure, the greater the light intensity, the shorter the exposure time; The smaller the light intensity, the longer the exposure time. If the light intensity is doubled, the exposure time needs to be reduced by half. If the light intensity is 2, the time is 4 and the exposure is 8; If the light intensity is 4 and the time is 2, the exposure is still 8. The concept of correct exposure

In order to achieve the correct exposure, the photographer must rely on two important devices of the camera itself to control the exposure, namely the shutter and the aperture. For photographic exposure, they act like two "shutters" that limit the passage of light: the shutter controls the time of light on the photosensitive film; Aperture is to control the amount of light reaching the photosensitive film. The former is to control the exposure time; The latter is to control the amount of light passing through the lens. If the shutter and aperture are properly matched, the required exposure can be obtained and the correct exposure can be achieved.

The so-called "correct exposure" is only relative. There is no absolutely correct exposure. There are three reasons: first, the tolerance of photosensitive materials cannot accommodate this difference in nature. The maximum brightness contrast of natural scenes can reach 1: 10000, and the brightness contrast of scenes usually contacted by photography is about 1:200. 1: 128' s film latitude is doomed to be unable to accommodate it, and 1:30' s photographic paper latitude can't do anything about it, so it has to lose a considerable part of its hierarchy. To make up for this, we need to solve it from the aspects of light illumination and exposure control. Secondly, the concept of "correct exposure" will vary from scene to scene and from person to person in terms of lighting, shooting skills, picture effects and other artistic expressions. In order to pursue different artistic effects, we can use the same equipment to face the same scene, and if we choose different exposure combinations, we can shoot two works with completely different effects. Third, post-processing factors have a significant impact on exposure.

Therefore, when mastering "correct exposure", we should consider both technical factors and artistic factors. In short: Never forget the application value and artistic factors of "correct exposure" in photography practice. ② Equal exposure and selective exposure

The purpose of photographic exposure is to get a satisfactory picture, but it has a lot to do with the choice of exposure combination. ● Equal risk

We clearly understand a simple truth. A large aperture allows more light to pass through the lens to reach the film than a small aperture, and a low shutter speed allows light to stay on the film for a longer time than a high shutter speed. It can evolve into the following situation: the combination of aperture f/ 1.8 and shutter speed 1/500 seconds, and the combination of aperture f/4 and shutter speed1100 seconds, the exposure effect is the same. They are also consistent with the exposure effect obtained by the middle aperture f/8 combined with the medium shutter speed of 1/25 seconds. This is called equal exposure.

The exposure of the above three groups of exposure combinations is equal, the density reflected by the negative film is the same, the tone level reflected by the black and white photos is the same, and the tone restoration and reproduction reflected by the color photos are also the same. However, there are two important differences in the picture effect: first, the size of the aperture forms different depth of field; Second, the definition of moving images is different because of the shutter speed. ● Select exposure

From the principle of equal exposure, we know that equal exposure with different exposure combinations can obtain the same negative density exposure effect. However, in photography practice, we should choose the best exposure combination among several exposure combinations that can produce the same density according to the difference between the subject and the subject to express the shooting theme. This is the concept of selective exposure.

Under certain conditions, beginners can get "satisfactory photos" by using a certain exposure combination (assuming f/8,1100 seconds). This "unexpected gain" has prompted some enthusiasts to regard it as a "universal exposure combination", and they often cling to it in their future photos, regardless of the occasion and without seriously considering different situations.

Those experienced photographers know from the theoretical and practical experience of photography that f/8,1100 second is only one of several exposure combinations that can be selected, and there are many combinations to choose from. Although each shutter and aperture can produce the same exposure effect, the depth of field and the definition of moving objects in the picture are quite different. Careful consideration must be given to which exposure combination to adopt. Is it possible to get a farther and clearer scene range with a smaller aperture? Or use a higher shutter speed to capture the "frozen image" of the disturbed object? Or use a very short depth of field to blur the background and highlight the subject? Or use a slower shutter speed to enhance the motion of the moving body? Wait a minute. All this requires the photographer's brain to think and choose, which is by no means solved by those "automatic cameras" because "automatic cameras" have no thinking ability. Photographers with image thinking ability can easily operate the camera to complete their own ideas, which may be the difference between the "automatic exposure" of the camera and the active "selective exposure" of people. Reciprocity law and its failure

In photographic exposure, we often encounter problems about reciprocity law and its failure, which must be understood and mastered from both theoretical and practical aspects. Reciprocal law

The definition of exposure tells us that all films with the same 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 daily 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.

In photography practice, the light sensitivity of the photosensitive film is generally limited to a certain range to produce normal exposure effect. If it exceeds the specified range, it is impossible for the latex layer to strictly abide by the exposure law. When the illumination of the photosensitive layer of the photosensitive film is too large or too small, or the exposure time is too fast or too slow, the exposure with the same amount may produce different exposure effects. For example, when the light is weakened to a very low degree, even if the exposure time is relatively increased, the satisfactory exposure effect may not be obtained; On the contrary, when the light intensity is extremely bright, shortening the exposure time to a very short moment with a very high shutter speed may not necessarily achieve a satisfactory exposure effect; On the contrary, when the light intensity is extremely bright, shortening the exposure time to a very short moment with a very high shutter speed may not necessarily achieve a good exposure effect. The above two phenomena are called light exhaustion. For example, when shooting the starry sky with weak brightness in celestial photography, or shooting with electronic flash in a very short moment, there will be the phenomenon of light exhaustion. Different photosensitive materials have different degrees of light attenuation, and color film is more prone to light attenuation than black and white film. This phenomenon is caused by the reaction inertia of photosensitive film to light. The light is very dark or the exposure time is very short, so it is difficult for the light to penetrate the inner layer of emulsion, which reduces the photochemical effect of the film. Assuming the same film is used, the illumination is 1000 lux, and the exposure is 1 sec, which is higher than the silver shadow density generated by the illumination is 1 lux and the exposure is 1000 sec. This phenomenon is the failure of the law of reciprocity. Failure of low reciprocity law

The concept that reciprocity law fails in photographic exposure is that the same exposure acts on the same photographic film, resulting in different densities.

In the process of photography, in the shooting stage, in the darkroom amplification stage, in black-and-white film and color film photography, the phenomenon that the reciprocity law is effective is often encountered. At dusk and dawn, and in the case of dark indoor light, the exposure time is very long, which often leads to underexposure or color cast.

The failure of reciprocity law should be grasped in photographic exposure. When using all kinds of films, we should fully understand their performance, especially pay attention to the reciprocal law exposure increment data provided by film manufacturers. For example, Kodak 35 mm film in the United States was shot in dim light. When the exposure time measured by the exposure meter is 1 sec,1sec and 100 sec respectively, the actual exposure time should be 2 sec, 50 sec and 1200 sec respectively. On the contrary, when shooting under high-speed electronic flash, if the exposure time measured by the exposure meter is110000 seconds, it is necessary to increase the aperture by half a step compared with the normal exposure.

In order to meet the different needs of photography and solve the problem of the invalidation of reciprocity law, some foreign photosensitive material manufacturers have specially produced films suitable for long exposure and short exposure respectively. The long exposure film is marked with L, which is suitable for exposure from110 seconds to 120 seconds or longer; The short exposure film is marked with the word S, which is suitable for exposure from110 seconds to 1/2000 seconds. ④ Exposure and negative density

The purpose of photographic exposure is to obtain a negative with normal density. Therefore, to understand what density is, we should also grasp the influence of density on image effect. The concept of density

Density refers to the number of silver particles deposited per unit area of negative film after exposure, development and fixation of photosensitive materials, which is used to indicate the degree of blackening. This black color is usually called density.

In the process of exposure and development, the silver halide in the emulsion layer of photographic film is reduced to different amounts of metallic silver according to the brightness of the photographed scene. Metallic silver is an opaque granular substance, which can absorb and block light. Explained from the brightness of the scene, when the shot scene is illuminated by strong light, its backlight intensity is stronger. After photographic exposure and development, the brighter the part, the denser the reduced silver particles, the more silver particles deposited and the lower the transparency. It is observed that there is more light blocked by shading and less light transmitted. On the contrary, the darker the part, the more dispersed the reduced silver particles are, the less silver particles are deposited, and the higher the transparency is. It is observed that less light is blocked by the shade and more light is transmitted.

Different amounts of silver particles deposited on the negative film constitute images with different blackness. Because density has different degrees of shading ability, it can be determined according to the amount of transmitted light. ● Density and image effect

From the perspective of photographic sunlight, the greater the exposure, the overexposure of the film, the more silver particles deposited, the deeper the blackness of the image and the greater the density of the film. On the contrary, the smaller the exposure, the lower the sensitivity of the film, the less silver particles are deposited, the lighter the blackness of the image and the smaller the density of the negative film. It can be seen that the exposure directly affects the density of the film. Neither overexposure nor underexposure can obtain the "ideal density", which can only make it lose the level of light or dark.

The perfect image effect depends on the negative density produced by correct exposure. Any overexposed or underexposed film is difficult to print photos with ideal image effect. The overexposed negative has a deeper blackness and higher density in the bright part of the scene, and the printed photos and images can only show some shadow lines and layers in the shadow part of the scene to a limited extent, while the shadow lines in the bright part are white without hierarchical texture. Under-exposed film, the brightness of the scene is greatly lost, and the film is generally light in blackness, low in density and transparent or translucent. Printed photos and images can only show some light and dark levels in the bright part of the scene, while the light and dark levels in the shadow part disappear into gray-black tones. It goes without saying that if you want to get the image effect with rich layers, normal contrast and bright tone, you can only rely on the negative with correct exposure and normal density. ⑤ Exposure and tolerance

In photographic exposure, the first step is to control the exposure so that the brightness of the photographed scene can be correctly reflected in the density change of the negative. In addition, there is another factor that affects the conversion from scene level to tone level, which is tolerance. The concept of tolerance

Tolerance refers to the ability of photosensitive materials to correctly record the brightness range of a scene in proportion. The difference between the largest part and the darkest part of the scene surface can be expressed by the proportion between light and dark. Suppose: the brightest part of the scene is 50 times brighter than the darkest part, then the ratio between them is 1:50, which is the brightness difference of the scene. The ability of photographic film to correctly express the difference between light and shade of the photographed scene is the tolerance of photographic materials, which is commonly called exposure latitude.

The tolerance of black and white film is1; 128 or so, the tolerance of color negative film is about 1: 32 ~ 64, the tolerance of color reversal film is only about 1: 16 ~ 32, and the tolerance of photographic paper is about 1:30.

In photographic exposure, using a photographic film with large tolerance to shoot scenes with small brightness difference has little effect on film density even if the exposure is a little more or less. From a practical point of view, the greater the tolerance of photosensitive film, the better the exposure control. ● Relationship between tolerance and exposure

There are three cases about the relationship between exposure and the width of photographic film: the width is equal to the brightness range of the scene.

When the width of the photographic film is equal to the ratio of brightness to brightness of the scene, the brightness range that the photographic film can record is exactly the same as the distance from the brightest to the darkest of the scene, so there is no other choice in exposure. If the exposure is slightly different, the level of bright or dark parts will be damaged. The tolerance is greater than the brightness range of the scene.

When the width of the photographic film is greater than the light-dark ratio of the scene, the range that the photographic film can record exceeds the distance from the brightest to the darkest of the scene, leaving a great choice in exposure. Usually, the lowest exposure is selected as the exposure standard within the allowable range, so that the density of the negative film can reach the minimum exposure standard, and the best image effect can be obtained by developing and playing photos or copying positive films. When the film width is larger than the brightness range of the scene, even if the control of photographic exposure is slightly different, it will not affect the reproduction of the light and dark levels of the scene. The tolerance is less than the brightness range of the scene.

When the width of photographic film is less than the brightness ratio, the brightness range that photographic film can record is far less than the distance range from the brightest to the darkest, so it is difficult to control the exposure and record the brightness ratio completely. Generally speaking, at this time, we should choose the focus of exposure according to the different conditions of the subject, take the brightness of the main part of the subject as the benchmark of exposure, and take into account the level of other parts. Obviously, the exposure in this case will inevitably lose a considerable part of the brightness range. The key is to properly determine the exposure focus and only lose some levels of the secondary part.