What is selective exposure?

With the appearance of exposure meter and automatic camera, it becomes easier and more convenient to choose the appropriate exposure. The exposure meter can measure the intensity of the light shining on the object and display its reading at the aperture and shutter speed. Exposure meters can be basically divided into incident type and reflective type.

The incident exposure meter measures the amount of light directly projected on the subject, which is very convenient to use. Just put the exposure meter in front of the subject and aim the semi-transparent hemisphere of the watch at the lens of the camera, and you can follow the instructions for exposure. The biggest disadvantage of the incident exposure meter is that it can't measure the amount of light reflected by the main body, which is the main amount of light acting on the film. In addition, using this exposure meter, sometimes the photographer can't get close to the subject for photometry, and sometimes it is necessary to take care of multiple subjects, some of whom are in the shadow, so it is difficult for the incident exposure meter to play its role. A reflection exposure meter measures the light reflected by an object. The built-in exposure meter of a camera is usually reflective. When in use, just aim the exposure meter at the direction to be measured. But it also has some problems. Most reflective exposure meters measure with a wide angle. If the surroundings of the subject are particularly bright or dark, the exposure meter will be "fooled" and provide wrong exposure information, resulting in underexposure or overexposure of the subject.

At present, although some reflective exposure meters can measure a small range, and some cameras have built-in exposure meters, they also have this "key" measurement performance, which can avoid the errors caused by the general reflective exposure meters when the background reflection is too bright or too dark, but users still have to use their brains.

Suppose there is a black subject, a middle gray subject and a white subject in a picture. If the reflective exposure meter is used to measure the gray subject, it will provide a certain exposure reading, such as L/ 125 seconds, F8; If the white subject is measured again, more light will be reflected on the photometer, so the exposure meter provides too little exposure, for example,1125 seconds, f16; ; If we measure black subjects, we will suggest more exposures, such as1125 seconds, F4. In this case, which data should be selected as the correct exposure reading of this picture? Experience has proved that exposure readings with gray themes should be selected. This is because the adjustment of the exposure meter is based on the intermediate gray scale. No matter what the source of the reflected light is, as long as it is taken according to its exposure reading, the photos obtained will be neutral gray tone.

If the exposure is based on the reading of the gray subject, the gray subject will appear gray in the photo, the black subject will appear black in the photo because of less light, and the white subject will look brighter and whiter because of more light.

If the exposure is based on the reading of the black subject, the exposure of the whole picture will be increased by two levels compared with the exposure provided by the gray subject, so the black subject will appear gray in the photo, while the gray subject and the white subject will become very white and bright due to the reflected light, thus overexposing the whole picture. Similarly, if the exposure is carried out according to the reading of the white subject, the whole exposure will be reduced by two levels compared with the reading provided by the gray subject.

Aperture, therefore, the white subject in the photo becomes gray, while the gray and black subjects will appear black or dark gray due to less light, and the whole picture is underexposed. Therefore, when using a reflective exposure meter, if you want to get the correct exposure, you should spend a little time thinking about determining the main body of the middle tone in the picture, and use it as a photometric benchmark, and then measure the darkest and brightest parts of the picture and compare them with the middle tone to know whether their levels can be preserved.

L Jacobs, an American photographer, summed up three situations in which exposure meters may cause metering errors, and put forward a solution: "The first situation is backlight. The background light intensity, the main body is dark, and exposure according to the exposure meter will cause underexposure. At this time, you can manually adjust the aperture, just open the aperture or slow down the shutter speed, or increase the auxiliary lighting.

"The second situation is that the background is bright. The subject is also bright. For example, people who wear white clothes in front of white walls or in snow-covered yards. If the exposure is made according to the reading of the exposure meter, it will be insufficient. The reason is simple, because the exposure meter will treat all this white as medium gray with a reflectivity of 18%. Of course, no one wants to make the white snow gray, so the aperture should be increased by one level, or the speed should be slower. Some photographers often deliberately underexpose when shooting snow scenes, thinking that they can shoot snow scenes well, but they are not. " The third case is that the background of the stability coefficient is dark and the main body is dark. At this time, the treatment method is opposite to the first case. There is a dark tablecloth on the table with a book with a black cover on it. If you follow the instructions of the exposure meter, the tablecloth and the book will be gray, so you should lower the aperture and increase the shutter speed by one level. "

Although modern automatic cameras are fully functional and easy to operate, the most advanced through-the-lens metering system can even cope with difficult situations such as backlight, but choosing exposure is still a skill. It involves the photographer's personal preference and style, rather than being replaced by a precise technology.

Photographer John Evans thinks: "Manual metering still has great potential. In addition, manual operation enables you to actively control the exposure. " Evans pointed out: "Manual mode means that the photographer decides the aperture and shutter speed. When shooting, if the light or the subject changes, one or both of them must be adjusted manually.

"Film speed, light intensity, shooting moving objects, preventing camera vibration and depth of field will all affect the choice of shutter speed and aperture. Use is0100/2121in bright light. Film photography, taking F8 typical exposure1125 seconds as an example. When choosing aperture and speed, we need to consider the vibration of the camera, the motion state of the subject and the depth of field.

"If you use a 50mm standard lens, the shutter speed of 3/ 125 seconds can completely prevent the camera from shaking, but if you use a 200mm lens, the image may be blurred, and the shutter speed of1.125 seconds will not solidify the fast-moving subject like a car. The lens is 28-200mm, F8, and the depth of field is normal. But some images need to be clear, deep and bright, shallow without losing the atmosphere.

"In order to meet these special requirements, the exposure of F8 and I/ 125 seconds needs to be adjusted; Select F4 and 1/500 seconds to prevent the camera from shaking; For curing, choose F2.8,11000 seconds; For the maximum depth of field, choose F22,115 seconds. " The exposure in each of the above combinations is equal, because any change in shutter speed is compensated by the corresponding aperture change: 1/500 seconds is1125 seconds, F4 is four times that of F8, and so on. This principle is called the law of equivalent exposure, which is often misused as the law of reciprocity. "

Evans also pointed out: "It must be understood that the correct exposure is a combination of appropriate shutter speed and aperture. This is also the basic principle behind all automatic exposure modes of aperture priority, shutter speed priority and various programs, and manual exposure can provide freedom of choice. "

John wilmot, a British photographer, said: "Generally speaking, all kinds of exposure are a compromise, but the correct exposure of the most important part of the image can often greatly improve the quality of the photo."

Wilmot pointed out that when taking photos, "whether you are near the subject for photometry or use a spot photometer, you can make exposure according to shadows or strong light, thus producing different results. In order to accurately determine which part of the exposure image, you need to have a keen eye and experience. "

Exposure according to the bright part will make the bright part show details, but the shadow part will be very dark or even very dark, and the whole photo will appear underexposed. For example, the sun shines on the sea, and the brightness is several orders of magnitude different from the foreground. At this time, panoramic exposure may make the foreground slightly underexposed, and most of the sunlight shining on the water will be lost. Reducing exposure will highlight all those dots and help show the details of the sky. This will darken the foreground, but will increase the overall effect. Wilmot believes that appropriate reduction of exposure can sometimes achieve good results. For example, if you shoot a white statue with a tree as the background, it is appropriate to take an average exposure, but if you lower the aperture by one level, bright objects will really stand out from the dark background.

Sometimes, because the bright sky affects the reading of the exposure meter, short exposure may damage the simple scenery. Wilmot thinks this problem is easy to solve, and the way is to make the camera lens slightly downward when measuring light. When there are particularly bright parts in the picture, such as bright buildings, beaches, water or dark objects on a light background, the photos taken are generally too dark. In order to obtain an acceptable effect, it is necessary to exclude the brightest part as much as possible when measuring light.

Wilmot believes that it is also possible to control the exposure to make the picture show different shades. High-profile can increase the elegance and lightness of the picture, while low-profile may bring dramatic effect and heavy atmosphere. High-light illumination should be fairly uniform to avoid shadows, and exposure should be used to get as much detail as possible in bright parts, which usually means a little overexposure. It is best to use side light for low-key photography, which is easy to cause a small amount of strong light and a little underexposed.

As for the color density of photos, it can also be obviously improved by slightly adjusting the exposure. For color negatives, accurate exposure or slightly overexposed scenes can obtain excellent density. Underexposure tends to make the color dull, while overexposure tends to make the color too dark and light. The same is true for color reversal films, but on the contrary, underexposure may make the colors thicker and brighter.

British photographer T Woodcock introduced the skills of handling objects in Gao Fancha, especially those exposed to strong light. The key is to try to achieve the highest reading and the lowest reading. First, measure the readings of the bright light and shadow respectively. When measuring, pay attention to make the bright light or shadow occupy the photometric range, and then expose according to the value between the two readings. For example: strong light reading: 1/500 seconds, F8; Film reading: 1/30 seconds, F8 exposure value:1125 seconds, F8. If the subject is too small, much darker or brighter than the background, then be careful not to be overly influenced by the background when taking readings. The method is to get close to the subject or measure light with a telephoto lens. The value thus measured is usually called a "critical" reading. If the object is moving or inaccessible, it is feasible to measure the reading of the substitute. The most convenient and commonly used method is to take a reading of the back of the hand.

Woodcock thinks that the problem of backlight should belong to the category of excessive contrast. The easiest way to shoot this kind of scenery is to readjust the picture, prevent the light source from appearing in the picture, ensure that there is no glare on the lens, then measure the readings, re-compose the composition, and increase the exposure by one and a half to two and a half steps. Another more accurate method is to remember the normal reflected light reading and then measure it from the back of the hand; Another reading, according to the exposure in the middle of the two readings.

Woodcock also summed up several techniques to achieve correct exposure.

1. Before reading, see if you can estimate the exposure correctly.

2. Pay attention to saving batteries at ordinary times. If you find that the exposure meter is broken and forget to bring the battery, you can adjust the shutter speed to the reciprocal of the ISO value of the film sensitivity, for example, the film is 1s0 100/2 1. Just adjust the shutter speed to1100 seconds, and take photos in sunny days, with negative F 16 and negative F 1 1.

Candlelight will seriously affect the exposure meter, so the lens should be properly shaded.

4. Try to use the telephoto lens as the measuring point 1 1 exposure meter.

5. When the exposure meter can't display the reading in dim light, you can turn the film sensitivity index dial to increase the ISO value until the exposure meter displays the reading. Then increase the exposure value, such as opening the aperture. Every time the ISO value of sensitivity is doubled, the aperture will be enlarged by one level.

6. When taking pictures with a super wide-angle lens, you must first measure light with a standard lens, and then shoot with a super wide-angle lens.

7. When using the reversal film in dim light, the exposure should be less than half a file to one file to keep the color saturated.

Freeman Patterson, a famous Canadian photographer, pointed out that the way for beginners to learn correct exposure is to carefully study the tone of the subject, so that they can make a correct judgment at a glance in various situations. We should be good at distinguishing what is the effect of being above the' average brightness' or reaching the first, second and third grades in the composition.

This requires repeated practice. Even after a lot of practice, you may not be able to master it skillfully, but it can always help you understand the instructions of the exposure meter, and understanding is only the first step to achieve correct exposure. He said: "If you can really identify the amount of light reflected or received by different parts of the subject, then you may change the lens or aperture, so that the exposure can reflect your feelings about the subject."

In short, the correct exposure of the subject depends on both subjective and objective factors. We should learn how to identify or estimate the tone or contrast of the subject, and then adjust the shutter and aperture appropriately according to the data of the exposure meter to make the tone brighter or darker, so as to obtain the expected exposure effect. Photographers always need to choose, and the correct choice depends on thinking and practice.