In short, the current frame cutting is to "equivalently" convert the range of pictures shot by sensors of different sizes into the standard of 35mm film (that is, Quan Huafu) through a conversion coefficient called cutting coefficient. The reason for this is that in the film era, 35mm film is widely used, and people in the industry can basically judge the angle of view and the corresponding imaging characteristics of lenses with different focal lengths in this specification. Gradually, people virtually convert angle measurement (field of view angle) into linear measurement (lens focal length). In the transition period to the digital age, due to technical and cost problems, the initial digital camera sensor size is basically smaller than 35mm film, so when using the same lens, the picture taken by the digital camera is only equivalent to the central part of the picture taken by the film camera, and the picture range is obviously smaller, so people call this change frame clipping, which probably means the central part cut from a complete photo. Later, in order to facilitate the unified calculation of the lens perspective, people widely used this so-called cutting factor. Of course, in fact, Quan Huafu is not the largest size frame specification, so some cutting factors are less than 1, for example, the cutting factor of 4433 middle frame is about .8, at this time, the cutting factor has lost its original meaning, just for the convenience of calculating the lens angle of view.

Digital cutting is basically frame cutting, but sometimes digital cutting is active, that is, for some needs, all sensors are not used, but only a part of them are used. Typically, the camera often involves digital cropping when shooting video. Due to the limitation of picture ratio and processing performance, the camera often does not use all sensors when shooting video, which leads to digital cropping. There is also the use of cropping effect to change the viewing angle. Smaller sensor size can obtain a smaller viewing angle without changing the focal length of the lens. Therefore, sometimes it is necessary to shoot with a telephoto lens in photography, but when there is no suitable lens at hand, you can actively abandon part of the picture through digital cropping in exchange for an approximate telephoto effect. This is sometimes called extended optical zoom technology. Of course, in essence, digital cutting or extended optical zoom technology does not change the focal length of the lens at all, because the focal length is the physical distance from the optical center of the lens to the imaging plane (which can be film, CCD or CMOS now).

optical zoom, as mentioned above, the focal length is actually the distance from the optical center of the lens to the imaging plane, so optical zoom is the zoom realized through the optical structure, which can be regarded as changing the focal length in a real sense. Usually it is achieved by moving some lenses in the lens. Simply speaking, the greater the optical zoom factor, the farther the scene can be photographed.