What is photometry?

Photometry is the process of measuring proper exposure. Only by measuring the correct exposure can we take satisfactory photos.

First of all, we should understand three concepts related to photometry.

Exposure: film or digital photosensitive element (CCD, etc. ) receives light from the lens to form an image. When we take a photo, if the scene in the photo is too bright, the bright part has no hierarchy and details, which is overexposure; On the other hand, the photo is dark and can't really reflect the color of the scene, that is, it is underexposed.

Correct exposure: What is "correct exposure"? Correct exposure is relative. Under the same lighting conditions, the light and dark parts of an object have different reflectivity. If you want to use film (or CCD and other electronic photosensitive devices) to correctly represent objects, the exposure of light-colored parts and dark-colored parts is also inconsistent. That is to say, in the same shooting range, as long as the reflectivity of the object is not used, there must be underexposure or overexposure. So in this case, as long as the subject we want to show is exposed correctly, this photo can be said to be exposed correctly.

Nowadays, cameras basically have automatic metering function, which allows us to take photos with appropriate exposure in most cases. However, in some cases, there will also be films with unsatisfactory exposure. It is often a scene that our human eyes can see. After taking photos, there will be a big gap. What is the reason? Sensitivity tolerance

We also need to understand a concept: photosensitivity. From the brightest to the darkest, assuming that the human eye can see the following range (see figure 1), the range that the film (or electronic photosensitive devices such as CCD) can display is far less than that seen by the human eye, and this limited range is the sensitivity tolerance. After understanding this concept, it is not difficult for us to understand why the human eye can see buildings (dark) and dazzling sky clouds (bright) with backlight. Once photographed, either the clouds are brightly colored and the building becomes a black silhouette, or the architectural color details are clear and the originally beautiful clouds turn white.

Figure 1, sensitivity tolerance

Figure 2. Because the contrast between light and dark is too large, while the clouds are correctly exposed, the building is seriously underexposed and has no color details, which becomes a black silhouette. (But this is exactly what this picture wants to show, so despite the serious underexposure of the building, this picture can still be said to be a "correctly exposed" film. Using the principle of "sensitivity and tolerance", the clouds are exposed correctly, and the building is made into a silhouette, which makes the film have a mysterious and dignified feeling. If you want to show the architectural color details, the sky will be completely white.

It can be seen that reasonable use of exposure can create photos with different feelings.

Metering mode: After understanding the previous "exposure", "correct exposure" and "sensitivity tolerance", let's learn about the camera's metering mode. Now SLR cameras and digital cameras basically have the following three metering modes:

1. average photometry: calculate the average photometry value of the whole scenic spot.

This is the default basic metering mode of general cameras, and the usage rate is the most common. You can almost take a satisfactory photo when the light is even and the contrast between light and dark is not great within the framing range.

2. Center average photometry: calculate the photometry value of 10-30% framing range.

When the subject to be represented is in the middle part of the framing range, and the ambient light and shade are quite different from the subject, the center average metering is selected, and only most central areas are metered, which can make the exposure of the subject more accurate.

3. Spot metering: also known as key metering, it is metering in the area of 1%-5% within the framing range.

Spot metering mode is rarely used and is not easy to master. But in some cases, spot metering can play an important role. Knowing when spot metering should be used and using spot metering correctly can make the exposure of the subject accurate on the one hand, and create wonderful pictures that can't be seen in reality by using "sensitivity tolerance" on the other hand.

Here are some examples to illustrate how to make good use of the spot metering mode.

Application of spot metering mode: spot metering mode is suitable for the situation of uneven light distribution and large contrast in framing. In this case, if spot metering is not used, the exposure of the object to be represented may be incorrect, too bright or too dark for white to have details.

Exposure lock: Before using spot metering mode, you need to know how to use the exposure lock. The metering point of the camera fixed-point metering is in the center of the scenic spot. When using spot metering, it is necessary to aim the center point of the scenic spot (the metering point of spot metering) at the subject to be represented to measure the light. If the subject is not at the center of the desired view, the metering point of spot metering can be aimed at the subject for metering first, and the data of the metering subject can be locked by using the exposure locking function of the camera. Finally, according to your own ideas, re-composition, focus and press the shutter.

Most cameras have a special exposure lock button, which is convenient for us to reconsider the composition after exposure lock. However, there are also some cameras whose exposure locking and focusing are achieved by half-pressing the shutter. If the metering point is inconsistent with the focus point, for example, in Figure 2, the metering point is a cloud on the sun side, while the focus point is a building, it is necessary to first measure the metering point, then lock the exposure, and then re-compose the focused building. Some cameras cannot provide separate exposure lock and focus lock. You can also record the exposure data after turning on the subject, then set the camera shooting mode to M file, set the exposure data according to the turned-on data, and then focus on the composition.

Figure 3. From the perspective of framing environment, the main body of the lotus flower that needs to be shown is bright, while the lotus leaf is dark, and the unevenness of the stem affects the composition. If we measure the light according to the average value, the average measurement value will be biased towards the light intensity in the dark environment, and the result of shooting is that the mosaic and pond are exposed correctly, while the main lotus we want to show is really pale (overexposed).

When shooting this film, lotus leaves reflect more light, while lotus leaves and ponds reflect less light. By using the spot metering function of the camera, the petals of the main lotus can be exposed correctly, highlighting the good colors and details of the lotus, while the pond is underexposed, and the dark part covers the layers of stems, which highlights the main lotus better.

Figure 4. This is a bicycle that is put indoors. Probably in the afternoon, there was no light in the room, and the sun shone in from the outside. Within the framing range, the light distribution is extremely uneven and the contrast is relatively large (the bicycle tires and the background behind the car can be clearly seen by the naked eye).

When shooting, click the button to measure the brightness, lock the exposure and re-compose. Due to the strong reflection in front, the background is almost pure black, which makes the picture quite simple and highlights the local lines and radians of the bicycle. In fact, this is also the charm of photography. Although what it reflects is true, it is different from what it really sees because of the sensitivity of film (or CCD, etc.). ) It's too narrow. It can be said that if you don't use a camera to shoot, you can't see such an effect with the naked eye.

By understanding the sensitivity, we can predict the shooting effect and extract this interesting film from the messy and ordinary scenery by spot metering.

Figure 5. Pay attention to metering when shooting people. The most important thing in portrait photography is usually to show people's faces.

In this picture, natural light is used to shoot the face, so that the face is exposed correctly, and the surrounding environment and clothes are darkened due to insufficient exposure. The picture is simple and the characters are prominent, which is similar to the feeling of background cloth used in the studio. If the light is measured evenly, the surrounding environment will be clear and the exposed face will be pale.

Reading this article will help beginners of photography:

1, learn about exposure, correct exposure, sensitivity tolerance, exposure locking and other photometry related knowledge;

2. Use the "spot metering" method to obtain accurate subject exposure.

By understanding and mastering the above basic knowledge, and then experiencing practical attempts and experience accumulation, I believe that friends who are beginners in photography can also take satisfactory and surprising photos.

Six Practical Photometric Methods in Photography Creation

In photography creation, the exposure situation is complex and changeable, and the color tone, brightness, atmosphere, quality and color purity of the subject are ever-changing, which is closely related to how to choose accurate exposure. Here are six photometry methods for reference.

First, the average photometry method

At the shooting point, the central key average metering function in the built-in metering device is used to directly meter the object, which obtains the average readings of various brightness within the measured scene. This method is beneficial to ensure that the whole film is properly exposed and the density of the whole film is neither thin nor thick. If the light and dark distribution of the subject is relatively uniform and the contrast is not great, it is easy to get good results with this average photometry method.

Second, take care of important shadow methods.

Using this photometric method, the hue of the important shadow parts of the most important object image in the picture can be accurately controlled, so that the hue and gradation of these parts are appropriate. When the shadow part of the picture occupies a very important part, such as the shadow part of the mountain, the backlit indoor scenery and the backlit portrait, this method should be used to determine the exposure. The specific method is: use a reflective exposure meter or the camera's metering system to measure the light in the important shadow part of the subject separately, but do not expose according to the reading directly indicated by the exposure meter, but reduce the exposure by 1, 2 or 3 levels compared with the exposure indicated by the exposure meter, so that the measured part can be displayed as a darker tone. In this case, the exposure should be reduced compared with the readings provided by the exposure meter. Black and white and color negative, the exposure is less than 3 levels lower than that indicated by the exposure meter, and the level can still be recorded; The exposure of color reversal film should be controlled within Grade 2.

Thirdly, consider the bright hue method.

This method does not focus on the tone of the dark part of the subject, but from the need to enhance or weaken the contrast of the scene, let the main scene light focus on why the tone should be reproduced, for example, shooting a close-up portrait with light or front light, or shooting a snow scene. This method will be used if you intend to reproduce the bright part of the face or the place covered by heavy snow as a middle tone or a brighter and brighter tone. Specifically, the reflective exposure meter or the photometric system of the camera is aimed at the bright part of the subject alone. After obtaining the photometric reading, the exposure is not according to the reading, but the l-2 level exposure is increased. When shooting high-profile scenes such as snow scenes, it is also necessary to increase the exposure in addition to the measured reading.

Fourth, master the brightness standard method.

This method is to measure the brightness of the bright and dark parts of the subject respectively, and then determine the appropriate exposure according to the width of the film. If you measure the bright side of the subject, you should use F 16 for exposure, while if you measure the dark side, you should use F4 for exposure, so that you can compromise with F8 in exposure. In this way, the bright surface exposure is more than 2 levels, and the dark surface exposure is less than 2 levels, so that rich layers can be recorded. According to the tolerance of black and white and color negative, as long as the exposure is not more than level 3 and the dark light is not less than level 4, the negative film is still layered. Generally speaking, if the negative film is developed normally, the brightness of black-and-white and color negative film is above level 3, and the brightness of dark part is below level 3, it can be layered. The range of tones that can be recorded by the color reversal film does not exceed 1:50.

Five, the gray board method

This method does not directly measure the brightness of the subject with an exposure meter, but measures the surface with intermediate reflectivity, and carries out exposure according to the measured reading. In this way, the surface with standard brightness (middle gray surface) in the shooting scene is reproduced as middle gray tone in the photo, and the darker or brighter surface will get darker or brighter tone than the middle gray. The most standard medium gray surface is the photographic test gray board, with a reflectivity of 18%. Put such a gray board in the position of the object to be photographed, and make it receive light evenly, then aim it with a reflective exposure meter for measurement, and expose it according to the measured reading, so that the object to be photographed can be exposed correctly. This photometric method is consistent with the result of brightness exposure meter. Because it can correctly reproduce the tone depth of light and dark parts in the shooting scene, it is especially suitable for shooting color reversal films.

If there is no gray board with reflectivity of 18%, the photographer can use an exposure meter instead of the gray board to measure the back of the hand, because their reflectivity is close. According to the brightness exposure of the back of the hand, it can also well reflect the light-dark relationship of each part of the subject.

VI. Method of Measuring Proxy Target

When the subject is far away from the camera and the local brightness near the subject cannot be measured, the method of measuring the substitute target can be adopted, that is, selecting a substitute target with brightness equivalent to that of the distant subject from nearby and directly measuring its reflected brightness instead of measuring the distant subject. For example, measure the nearby snow, not the equally bright snow on the distant mountain peak; Measure the trunk or leaves of a nearby tree, not the tree across the river. However, when using this photometry method, it should be noted that the light receiving conditions of the substitute target and the actual object must be consistent, and the background cannot affect its reading, so as to get accurate results.