Professional explanation of focus distance

The lens of the camera is a group of lenses. When parallel rays pass through the lens, they will converge on a point. This point is called the focal point. The distance from the focal point to the center of the lens is called the focal length. A lens with a fixed focal length is a fixed-focus lens; a lens whose focal length can be adjusted and changed is a zoom lens. In the field of photography, the focal length mainly reflects the size of the lens angle of view. For the traditional 135 camera, the angle of view of the 50mm lens is close to the human eye, and it is not distorted when shooting. It is called the standard lens and generally covers the range of 40-70mm. 18-40mm is called the wide-angle or short-focus lens, 70-135mm. It is called a medium-focus lens, 135-500mm is a telephoto lens, more than 500mm is called a telescope head, and below 18mm is called a fisheye or ultra-wide angle lens. This division of the range is just a habit of people, and there is no strict definition. The CCD of a digital camera is generally much smaller than that of a 135 film, so the focal length of the lens is much shorter at the same angle of view. For example, when using a 0.33 "CCD digital camera with a 13mm lens, the angle of view is roughly equivalent to the standard of the 135 camera 50mm Lens. Because the CCD specifications and models used by various digital camera manufacturers are different, everyone uses the expression "equivalent to the focal length of a 35mm camera (ie, 135 camera)".