What is exposure (photography)? What do you mean by repeated exposure?

Do we often encounter the following situations?

When we see an exciting scene-perhaps the snowy mountains in New England or the charming autumn colors, we often grab the camera and take this scene into the lens. However, we often get depressed images, and none of the color pictures in the original scene are captured. If we shoot with color film, what we see is that those beautiful bright colors have turned into dull pale colors. If you shoot black and white film, the photo will be full of dim and faded gray tones.

It may happen to all of us, and we always lose the bright colors and tones that we can see on the spot more or less inexplicably.

The crux of the problem is that we didn't expose the film properly. We either overexposed the scene or underexposed it. Even if our camera provides the most advanced and very simple built-in exposure meter, and even provides automatic exposure function, we will still make this mistake unconsciously. So, what's wrong with these extremely safe metering systems?

The answer is simple. There is nothing wrong with the exposure meter, but ourselves! That's true. The exposure meter will read anything pointed by itself. However, the key is to know where the exposure meter should point and how to understand the read data. If you understand this truth, you will know the trick. The smartest exposure meter in the world can't make perfect exposure by itself unless we, as photographers, aim it at the correct position of the scene and use its data flexibly.

When we expose a color film correctly, all the colors on the photo or slide should be bright and pure. Similarly, when we expose a black-and-white photo, all the colors in the final photo should be very bright.

The trick is to know how to expose the film correctly. This lesson will cover the following:

How to know where the exposure meter should point;

How to explain what the exposure meter shows?

In order to help fully understand these contents, let's start with the basic knowledge of how to expose black and white film. Once you know how to expose black and white film, the problem of how to expose color film will be solved.

Ok, let's talk about how to master the perfect exposure skills. First of all, we should know what happens when we expose a black-and-white photo. Then, transfer the problem to color film. As we know, color film is based on three layers of black-and-white emulsion, which is related to the three primary colors, and produces color images from the three primary colors. .

What happened when the black and white film was exposed?

We already know that when light is irradiated on the emulsion, the tiny crystals in silver halide will change, and when developing, the crystals changed by light will form black metallic silver crystals. Therefore, the frame area irradiated by light will become a dark black metallic silver area on the negative.

What about those areas in the frame that are not illuminated by light? The silver halide crystals in these areas remain unchanged and will be washed away during development. Therefore, the areas in the frame that are not illuminated by light will become blank areas on the negative. All that remains in these areas is the transparent acetic acid substrate of the film.

To better understand this content, let's look at two very extreme situations.

First, consider what happens when we expose the whole frame to bright light. For example, we accidentally turned on the camera and exposed the film to bright sunlight. What happens when developing such film? Almost all silver halide crystals will turn into black metallic silver, and the resulting negative is all black.

Secondly, consider what happens when we develop a film without any exposure. For example, sometimes we are not sure whether a film has been made, so we develop it for safety. If the film has never been exposed, you will get a whole roll of transparent negative-all the silver halide crystals are washed away during development, leaving only the transparent acetic acid film base.

These two extremes are easy to understand. Let's consider what might happen in real life. What happens when we expose a film while taking pictures? Strong light will fall on some areas of the film, weak light will fall on other areas, and some areas have no light at all, resulting in the following results:

In the process of developing, the area of film illuminated by strong light. Almost all the silver halide crystals on it will be converted into black metallic silver, and these areas are black on the negative.

During the development process, most (not all) silver halide crystals on the areas of the film irradiated by medium brightness light will be converted into black metallic silver, and these areas are dark gray on the negative.

In the areas where the film is irradiated by weak light, only a small amount of silver halide crystals are converted into black metallic silver during the development process, and these areas are light gray on the negative.

There are no areas irradiated by light on the film, and no silver halide crystals are converted into black metallic silver during development, so these areas are transparent on the negative.

In this way, each area on the negative film corresponds to the relative intensity of light in this area in the shooting scene. The negative film shows different degrees of metallic silver aggregation, from the completely black area covered with thick metallic silver to the slightly thick metallic silver area, until the transparent area with only acetic acid film base.

How to determine the appropriate exposure

We know that the gray-black tone on the negative depends on the degree of black and silver aggregation. The thickness produced by this aggregation is actually measurable. If we observe the cross section of the developing negative under a high-power microscope, we will see a series of peaks and valleys made of metallic silver, as shown in Figure 5. 1

The peak marked a is the area exposed to extremely strong light during exposure. When viewing the negative, it looks black. The deep valley marked with C is an area where there is almost no light, and it actually looks transparent. Area B is a light gray area irradiated by moderate intensity light. The higher the concentration of metallic silver, the darker the gray.

When we take a photo with this negative, the tone will be reversed. The black peak at A in the photo is pure white, the transparent deep valley at C is almost pure black in the photo, and the light gray area B is dark gray in the photo. Therefore, the photos bring us back to the original scene. That is to say, the bright area in the original scene is dark on the negative, but it becomes bright in the photo. The dark areas in the original scene are very bright on the negative and become dark areas in the photo.

The bright areas in the original scene are called bright parts, which are bright in the original scene, dark on the negative and bright in the photo. The dark area or black area in the original scene is called the shadow part, which is dark in the original scene, bright on the negative and dark in the photo.

The same change will happen on each layer of color film. Blue causes ferrous silver to gather on the blue photosensitive layer, green causes ferrous silver to gather in the green photosensitive layer, and red causes ferrous silver to gather in the red photosensitive layer.

Now, let's see what the relationship between all this and correct exposure is.

In order to determine the exposure, you need to use an exposure meter. There may be a built-in exposure meter on the camera, or you can use a separate handheld exposure meter. If the exposure meter is built-in, the camera often provides a convenient function of automatic exposure.

automatic exposure

At present, all camcorders and most SLR cameras offer the optional function of automatic exposure (AE). Theoretically, all we have to do is aim the lens at the subject and press the shutter button. The built-in exposure meter will do the rest, and it will automatically calculate the "correct" exposure whether using color film or black-and-white film. It seems perfect, but unfortunately the built-in exposure meter thinks that the "correct" exposure is often wrong!

Why? Because exposure meters can't think, only we can think. Should we correctly expose the subject's face, or just expose the sky and capture the gorgeous sunset by showing the outline of the face? AE metering system can only guess what we want, and even the complicated metering system can't know what we are thinking.

We know what we want to do. This lesson will learn how to use the camera's built-in metering system and a separate exposure meter to ensure that we can get the required exposure every time. We don't guess, but we know.

How to get accurate exposure: photometry

Exposure is the key to high quality photography. Accurate exposure and natural color reproduction. Too much exposure will weaken the color, the subtle color difference will disappear, and the light color in the scene will almost turn white, thus making the scene lose some color.

However, if the color reversal film is slightly underexposed (within half an aperture), the color will be more saturated and the colors such as red, green and blue will be more abundant. But if the exposure is less than half a step, the color will become too dark and impure: the light color will turn yellow-brown, and it is difficult to distinguish between blue and black. Exposure error does not have such a serious impact on color negatives, because color negatives can be corrected when developing. Nevertheless, if the image quality is not seriously damaged, the boundary between overexposure and underexposure of color negative film is only a stop point.

Black and white film has great tolerance, especially overexposure. Black and white film with accurate exposure shows a large range of light and shade. However, when exposed too much, it will not only lead to the weakness of the bright part or even a "dead white", but also turn the originally strong black into a dull dark gray. In addition, the picture particles become coarse and the details are destroyed, resulting in a rough picture, which is completely out of proportion to the main body.

The influence of underexposure on black and white film is usually much more serious than overexposure. It will cause the white part of the scene to appear gray again, and the black part will appear as atypical black. In photographer's jargon, the range of tones has narrowed and the dark details have disappeared.

-estimated exposure

Better more than less and step-by-step exposure are two common methods.

What kind of exposure mode is adopted can be changed according to the requirements of exposure accuracy. The most commonly used method is to estimate the exposure. Many professional photographers get excellent exposure results according to their own experience. The advantage of using estimated exposure is that dependence on photometric equipment can be avoided. Photometric equipment will fail or even fail, providing you with inaccurate exposure data. However, experience is experience after all and cannot be taught in advance.

The scene reference table using exposure reference table is one of the methods to estimate exposure. This exposure reference table recommends certain exposure combinations according to the film speed and some typical light conditions. It allows you to replace the exposure meter with your eyes, and the exposure effect will be quite close to that recommended by the accurate exposure meter.

This exposure reference table divides the lighting conditions into several types: beach or snow under cloudless or thin Yunyang light; Cloudless or thin sunshine (clear projection); Bright and cloudy (no projection); Severe cloudy day; Open and dark places (sunny days, but no direct sunlight) You can adjust the shutter speed and corresponding aperture according to the requirements indicated in this exposure reference table.

The trick to using this exposure reference table is not to be too blind. For example, it only points out the exposure data of a cloudless sunlight, which is aimed at the subject facing the sunlight. But the actual situation of shooting may not be like this (for example, taking portraits is not suitable for facing the sun). If the subject's back is to the sun, or the subject is partially in the shadow, it is necessary to increase the exposure if you want to show the details of the dark part.

* technical points for evaluating exposure under strong light:

When you don't have an exposure reference table or another light table, you don't have to give up the shooting plan. You can use a simple "falling film number method" to estimate the exposure.

Reciprocal method of film speed: shoot the subject from the front in bright sunlight, use fi6 as the aperture, and use the reciprocal of ISO/AS film as the shutter speed. For example, if you use ISO/ASAZ00 film, the exposure combination under strong light is 1/200 seconds, fi6. Because the camera does not have an excessive shutter speed of 1/200 seconds, you can choose a shutter speed close to its tit, that is, 1/250 seconds. The exposure combinations that are the same as 1/250 seconds and fi6 are 1/500 seconds, fil, L/ 1000 seconds, fs, L/2000 seconds, fs stone, etc. You can choose according to your own needs.

Taking this exposure data as a general guide, if there is no sunlight in front of the village, open the aperture a little; Open the second aperture on a sunny and cloudy day; Or open a third aperture in the shade. If you shoot in snow, beach or water with high reflectivity, you will lose a light day.

2. Measuring exposure with an exposure meter-accurate photometry

There are several different types of exposure meters, each of which has its own unique use value. If your camera has a built-in metering system, this may be the exposure meter you are using. The internal metering system of the camera is a reflective exposure meter. Of course, the reflective exposure meter is also made into a hand-held style.

Reflective exposure meter is aimed at the subject at the camera position to measure the light. When light shines on the main body, different bright parts and dark parts of the main body reflect different amounts of light. The photometric system of the camera measures this reflected light. In order to obtain accurate metering effect, you must know the measuring range of your camera metering system. The metering system of some cameras measures the average brightness in the whole picture; Others are mainly scenes in the middle and lower parts of the picture; Others only measure the scene in the central area of the picture, which is often the range of the central focus indicator circle on the viewfinder of most single-lens reflex cameras. Check carefully what kind of metering system your camera or hand-held exposure meter belongs to, so that you know how to aim at the subject accurately and get the metering reading you need.

The internal metering system of a camera is usually accurate, but it can also cause some problems. Sometimes the metering system can fool you. For example, if you shoot a snow scene in bright light, the reflected light will be enhanced by the snow, and the exposure of the reflective exposure meter will be less than the actual exposure. If the subject wears a white shirt and hat, it will also lead to underexposure. Because the surface of an object with high reflectivity reflects more light than it needs to display all the details of the scene, the photometric reading is not accurate.

Similarly, for dark subjects, it will also lead to inaccurate photometric readings. Because the surface of dark objects absorbs most of the light and the amount of reflected light is very small, the exposure data measured by the exposure meter is more than the required exposure. This will lead to overexposure of the whole picture and loss of bright details.

The incident exposure meter is a direct measurement of the light shining on the main body. When the incident exposure meter is used, the exposure meter is placed at the position of the main body, and the exposure meter reads photometric readings for the camera. Because the incident exposure meter only measures the light that irradiates the subject, it is not affected by the light around the subject. It is ideal to use the incident exposure meter almost every time you shoot a color film.

The incident exposure meter covers the measurement window with a soft opalescent lampshade. Common soft masks are like half a ping-pong ball, or like an inverted shallow cone, and some use sliding wavy shading screens to block the metering window. The function of these ivory lampshades is to receive light from all directions at the same time. Most hand-held reflective exposure meters can be used as manual emission exposure meters, as long as a soft light attachment is added to the metering window. Some manufacturers also provide soft light covering the lens, which can make the camera's internal metering system be used to measure the incident light.

No matter your experience, or the exposure reference table, or how the exposure meter thinks it should be exposed, there may be errors. Because when you look at a scene with your eyes closed, you can clearly see its details, whether it is the strong light or the darkest shadow under the bright sun. The film in the camera is much more limited. You should always consider what you want to record, and then expose it in a targeted way. Third, use photometric readings flexibly.

The reflectivity of an object is not always the same as the standard reflectivity used by manufacturers for exposure meters. When the reflectivity of the subject is obviously greater than or less than this%, if the reading light is exposed, the color tone of the subject cannot be accurately recorded.

In the usual shooting. Flexible use of photometric readings is required. The exposure meter regards all photometric measurement objects as having average reflectivity. When you measure the light of dark skin, the reflected light of dark skin is less than the average reflected light, but the exposure meter thinks that the light of the whole auxiliary object is not enough, so it tells you that more exposure is needed. This will lead to overexposure of the whole subject. If you want to record the natural characteristics of dark skin, you should greatly reduce the exposure on the basis of photometric readings.

When shooting on the beach or snow scene, because the reflected light of the surrounding environment is much more than the average light, the exposure instrument thinks that all the scenes are bright, so he tells you that you only need to expose less. This will lead to underexposure. In order to record the details and colors of the subject correctly, you need to increase the exposure on the basis of photometric readings.

The "error" of the above-mentioned exposure meter can be compensated by increasing or decreasing the exposure according to the photometric reading by step exposure method. But if you use "three-step" exposure, it means spending three times as much film; Moreover, for shooting moving objects, it is difficult to use step exposure method in both content and exposure. So you should be good at thinking and use laser reading flexibly to improve the success rate. * Determine the exposed technical points:

Here are some basic skills for accurately measuring exposure.

Color reversal films and color negatives can be used in any of the following ways:

1. Expose according to the reflected light reading of the most important color part in the subject, such as shooting people.

Objects can be exposed according to the photometric reading of skin color.

2. For ordinary scenes, exposure can be made according to the average photometric reading.

3. Expose by reading the incident light at the position of the object or according to the position of the object.

Exposure to incident light readings at other locations under the same previous conditions.

4. Expose according to the reflected light reading of 18% gray card, and the light receiving conditions of the gray card should be the same as the subject.

Same body.

Black and white negatives can be made in any of the following ways:

1. Measure the light in the darkest place of the important part of the subject, and reduce the reading according to this reflected light.

Secondary aperture exposure.

2. For dark red skin color, reduce the exposure by one aperture according to the photometric reading.

3. The skin color of yellow people can be exposed according to photometric readings.

4, for the usual scenery, according to the average photometric reading exposure.

5. Expose according to the reading of incident light at the position of the subject, or press the light receiving rod with the subject.

Expose incident light readings at other locations of the same component.

6. Expose according to the photometric reading of 18% gray card, and the light receiving conditions of the gray card should be the same as the subject.

Precautions:

1, when photometric reading is an important part of wave weight, the above methods are all applicable.

2. When the photometric reading is affected by a large area of dark environment, it is necessary to reduce the aperture from 1/2 to 1 on the basis of the above method.

3. When other light readings are affected by large bright environment, it is necessary to enlarge the light map from 1/2 to 1 based on the above method.

latitude

The ability of film to allow exposure errors is called tolerance. Step-by-step exposure is the use of film width. For color reversal films, less exposure is often more effective, which can increase color saturation and ensure the reproduction of bright details.

Repeated exposure

Repeated exposure is a relatively advanced function. Generally speaking, traditional cameras only have this function. Only a few higher-level models of digital cameras can be owned. Although it is not often used, it can create interesting special effects. Its principle is very simple, that is, the exposure compensation is one or two frames lower on the same negative. Don't shoot the negative first, and then shoot another one at another scenic spot, so that the synthetic effect can be naturally mixed.

In the photo below, the sky is slightly brighter, the sun has not broken through the clouds, and the moon is still high in the sky. Press the shutter at this time, because the moon is too high and too small, I always feel that something is missing; At this time, it is time to give full play to the function of "repeated exposure" for thousands of days!

With the popularization of computer equipment and post-processing software, this function can actually be replaced by computer synthesis, but using the principle of taking pictures according to the weather conditions at that time, this sense of accomplishment and pleasure is really incomparable to computer synthesis!

Unbelievable India

References:

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