There are several automatic metering methods.

As far as the metering system of automatic exposure camera is concerned, there are two kinds of metering: external metering and internal metering. The same is automatic metering, the general public camera is called EE, and the single-lens reflex camera is called AE. When we take pictures, we use program exposure, shutter priority, aperture priority and other modes to take photos with automatic exposure. Automatic exposure mode (1) external metering automatic exposure camera: the light receiving position of the photosensitive element does not pass through the lens, and the light receiving window of the metering system-electric eye can be seen on the external surface of the camera. Because of the photosensitive element. The reaction speed is slow, especially when the light changes from strong light to weak light, it takes several seconds to more than ten seconds to measure the accurate light value, and the field of view of the light receiving window is inconsistent with the lens imaging, which is easy to cause metering errors, but it can still meet the needs of ordinary amateur photography. Generally, there are two installation positions for the photosensitive element of external photometry. One is to install the CdS electric eye at the front end of the lens; The other is to install a CdS light receiving window in front of the camera top cover. Most cameras adopt the first installation position, which has the advantages of being close to the lens and having small metering error; When the lens is added with a filter, the filter blocks the electric eye at the same time, and the user does not have to consider the influence of the filter on the exposure. (2) Internal photometry automatic exposure camera: The light measured by its photosensitive elements (mostly SPD-silicon photocells, commonly known as "blue silicon") is the imaging light passing through the lens, which is referred to as TTL photometry. Photometric element "blue silicon" is sensitive to red light, so a blue filter should be added to correct it. In recent years, blue silicon batteries have come out. Blue silicon battery and blue silicon battery have fast response, good linearity and small dark current in low illumination, and are superior to cadmium sulfide photoresistor in spectral response, output characteristics and stability. There is also a new type of photosensitive element-phosphorus gallium arsenide, which has good output linearity, reliability and stability, and is more suitable for the actual situation of photography, and has been adopted by new models in recent years. The advantage of internal photometry (TTL) is that the photometry direction is consistent with the photography direction, and it is not easily influenced by the light outside the field of view, so it has high accuracy. After adding filters or other additional lenses, the exposure can be corrected automatically, especially after switching to zoom lens or telephoto lens, which is not affected by the change of visual field; When using ring and leather cavity for close-up, photometry is not only accurate, but also eliminates complicated calculation. Popular electronic cameras all use external metering; Single-lens reflective electronic cameras all use internal metering. The significance of automatic exposure The camera can automatically set the shutter speed and aperture value according to the exposure value of the shot picture measured by the metering system and the combination of shutter and aperture exposure set by the manufacturer during production. As far as camera operability is concerned, this method is equivalent to the so-called "fool camera". The operator does not need to adjust the shutter speed and aperture value at all. All he has to do is focus and press the shutter button. The common electronic program shutter of "fool" camera belongs to this exposure mode. In fact, only the program automatic exposure mode is the real "automatic" exposure mode. Auto-exposure proofs I found auto-exposure photos online, all of which were taken by netizens themselves. The results are still quite good, which is very helpful for you to learn photography.