What do you mean by focal length?

The focal length is the zoom range, which is the focal length range of the lens itself. The focal length represents the ability of the lens' visual range. The larger the focal length, the smaller the visual range, that is, the telephoto lens, also known as the telescope head, is often above 150 mm, on the contrary, a lens smaller than 35MM becomes a wide-angle lens. The focal length of SLR camera is based on the imaging range of 35MM ordinary film. If it is installed on a digital SLR, because the CCD imaging area of a common digital SLR is relatively small, there is generally a coefficient of about 1.5, that is, the lens of 18MM is equivalent to the visual range of the lens of 18× 1.5 = 27 mm on a digital SLR. The telephoto is longer, for example, it used to be 200MM, but now it is 200× 65438. The focal length of the lens itself will not change, but the visual range will change. For example, the focal length of a camera lens is equivalent to 18-200mm of a 35mm camera, which means that the imaging range of this lens on this machine is the same as that of a18-200mm zoom lens on a common film machine. The data itself is of little use for photography, and only provides a contrast to distinguish the degree of wide angle and telephoto. Each focal part has its specific visual range.