What is photometry?

The principle of camera automatic metering

How does the camera automatically measure light? In fact, the principle is very simple. The reflectivity of the metering area automatically erected by the camera is 18%, and then the values of the aperture and shutter are determined by this ratio. Aperture and shutter are related. Under the same lighting conditions, the larger the aperture value, the smaller the shutter value, and the smaller the aperture value, the larger the shutter value. The numerical source of 18% depends on the reflective performance of middle tones (gray tones) in natural scenery. If there are many white tones in the framing picture, the reflected light will exceed 18%. If it is an all-white scene, it can reflect about 90% of the incident light, while if it is a black scene, the reflectivity may be only a few percent. The standard gray card is an 8× 10 inch card. If this gray card is placed under the same measuring light source of the subject, the overall reflectivity of the measuring area will be 8% of the standard. Then, you only need to shoot according to the aperture shutter value given by the camera, and the photo will be exposed accurately. If the overall reflectivity of the whole metering area is around 18%, as we said above, the background is dominated by white tones. At this time, if you shoot according to the aperture shutter value determined by the camera's automatic metering, the photo you get will be an underexposed photo with a gray white background. If it is white paper, it will become black paper. Therefore, shooting scenes with reflectivity greater than 18% needs to increase the EV exposure compensation value of the camera, and the specific compensation EV value needs to be analyzed according to the specific situation. At this time, experience is very important. On the other hand, if you shoot a scene with a reflectivity lower than 18%, such as a black background, your photos will often be overexposed and the black background will turn gray. Therefore, shooting scenes with reflectivity below 18% needs to reduce EV exposure, which is the principle of "white plus black minus".

If the camera supports manual setting of exposure compensation, this method can be used. If the camera doesn't support this function, you can set it in the manual gear of the camera, record the aperture shutter value obtained by the camera's first photometry with the automatic gear, and then switch to the M gear, so as to achieve the purpose of exposure compensation by appropriately increasing or decreasing the shutter speed.

What is TTL measurement?

We can see a common term "TTL metering" in many camera specifications. What does this "TTL measurement" mean? The English full text of "TTL photometry" is Through The Lens, which means through the lens. When used for photometry, it means that it is a method of measuring light through camera lens, which is referred to as "TTL photometry" for short.

The technology of TTL photometry originated from 1964. At that time, people needed to bring an exposure meter when they went out to shoot, and then set the aperture value and shutter value of the camera before shooting. The whole process is complicated. And "TTL measurement" just solves this problem. When shooting, the photographer half presses the shutter and the camera starts the TTL metering function. The incident light is refracted through the camera lens and reflector and enters the built-in photometric sensor of the fuselage. The working principle of this photometric sensor is similar to CCD or COMS, which converts the optical signal into an electrical signal and then transmits it to the camera processor for operation, so as to get the appropriate aperture value and shutter value. The user presses the shutter completely, and the camera automatically shoots according to the aperture value and shutter value given by the processor. The biggest advantage of TTL photometry is that the luminous flux obtained by TTL photometry is the exposure parameter of standard film. If a filter is installed in front of the camera, the metering value obtained by TTL metering is different from that without filter. At this point, the user does not need to readjust the exposure compensation according to the filter installed in the camera, just press the shutter to take pictures.

Most digital cameras or traditional fool cameras have these metering methods: center average metering, center part metering, spot metering and evaluation metering. These photometry methods can basically cope with all the current shooting, but in the photo studio and some professional occasions or advertising shooting, photographers still rely on the value of the exposure instrument to shoot.

Camera metering mode

Most digital cameras or traditional fool cameras have these metering methods: center average metering, center part metering, spot metering and evaluation metering. These photometry methods can basically cope with all the current shooting, but in the photo studio and some professional occasions or advertising shooting, photographers still rely on the value of the exposure instrument to shoot.

First, the central key average measurement (or simply: central average measurement)

Central average metering is the most widely used metering mode, and almost all camera manufacturers regard it as the default metering mode. Central average photometry mainly takes into account that general photographers are used to putting the subject, that is, things that need accurate exposure, in the middle of the viewfinder, so this part of the shooting content is the most important. Therefore, the sensory elements responsible for metering will organically separate the overall metering value of the camera, and the metering data in the central part accounts for the vast majority, while the metering data outside the center of the screen plays an auxiliary role in metering as a small proportion. After the processor of the camera weights and averages these two values, the photographed camera photometric data is obtained. For example, Nikon's camera uses the central key to average metering, and the metering in the central part of Nikon's camera accounts for 75% of the total metering ratio (this ratio varies from brand to brand), and the metering data in other non-central parts gradually extend to the edge, accounting for 25%. In most shooting situations, central focus metering is a very practical and widely used metering mode, but it is not applicable if the subject you need to shoot is not in the center of the picture or under backlight conditions.

Central focus photometry is a traditional photometry method. Most camera metering algorithms pay attention to about 2/3 of the center of the picture, and consider the periphery. For photographers who are used to using central focus metering, it is easier to control the effect by using this metering method than by using multi-zone evaluation metering method.

Suitable for shooting purposes: personal travel photos, special scenery photos, etc.

Second, the central part of measurement (or: partial measurement)

Central part measurement and central average measurement are two different measurement methods. Center average photometry is a photometry method that focuses on the central area, supplemented by other areas, while the central part photometry only measures one area in the center of the picture, and the photometry range is about 3% ~ 12%. The central part metering mode is suitable for some scenes with complex light, and more accurate exposure is needed at this time. Using the central part to measure light can get photos with accurate exposure of the object. Central part photometry can be applied to some special harsh shooting environments, which can ensure that the camera processor can calculate the exposure required for the main performance object part in the center of the picture. This mode is most suitable for scenes such as stage, performance and backlight, but due to the rise of split metering (matrix metering) mode, this mode is becoming less and less common in cameras. Canon, on the other hand, insists on central metering (partial metering). Up to the latest EOS 30V film camera and EOS 20D digital SLR camera, 9% area partial metering has been designed, which can make cameras without spot metering function reduce the influence of light on the subject when shooting pictures under some complicated light conditions.

Partial photometry is to measure the light of a certain part of the picture. When the contrast between the subject and the background is strong, and the proportion of the subject in the picture is not large, this metering method is the most suitable; In this case, partial metering is more accurate than the first and second metering methods, and unlike spot metering, because the metering point is too narrow to make mistakes, which requires some metering experience.

Suitable for shooting purposes: accurate metering is needed under certain conditions, and the metering range is larger than that of spot metering.

Third, SPOT measurement (spot)

Although central average photometry (central focus average photometry) can fully express the light reaction of the whole picture, it also has many shortcomings. For example, when it is necessary to accurately expose small-scale objects, the center average photometry (center focus average photometry) is not so good, and even the center part photometry (partial photometry) sometimes has a large range. In order to overcome these shortcomings, some manufacturers have developed this spot metering mode to avoid the influence of environmental light sources on the main metering under complex lighting conditions or backlight conditions; The range of spot metering takes a small area in the center of the viewfinder as the exposure reference point, and the metering area of most spot metering cameras is 1% to 3%, and the camera takes the light measured in this narrow area as the exposure basis. This is a fairly accurate photometric method, but it is not so easy for beginners to master it. How to distinguish a metering point has become a skill to be learned. The photos taken by the wrong metering point are either overexposed or underexposed, resulting in serious exposure errors. Because of the skill of spot metering, it can also be used in macro shooting of digital cameras, which can make macro exposure more accurate. Therefore, people who like macro photography must study this metering method hard and initially choose the middle small area in the picture as the metering reference point. Spot metering is also a good weapon in portrait shooting, which can accurately expose parts of people (such as faces and even eyes).