What is the principle of SLR photography?

Single lens reflection. Cameras using this technology use a single lens to take pictures, and use it to take pictures. In the working system of SLR camera, the light passes through the lens and reaches the 45-degree reflector, and then refracts to the focusing screen above to form an image. Through the eyepiece and pentaprism, we can see the scenery outside in the observation window. When the shutter button is pressed to shoot, the reflector will bounce up, the shutter curtain in front of the photosensitive element (CCD or CMOS) will be opened at the same time, and the light passing through the lens will be projected onto the photosensitive original, and then the rear reflector will be restored immediately, and the image can be seen in the viewfinder. This structure of the single-lens reflex camera ensures that it is shot by focusing the lens, so that the image seen in the viewfinder is always consistent with the image on the film, and its framing range is basically the same as the actual shooting range, which is very conducive to intuitive framing and composition. This structure can easily replace lenses with different specifications (because no matter what lens focal length you see in the viewfinder, it is always consistent with the actual shooting), which is the inherent advantage of SLR cameras and incomparable to ordinary digital cameras.