The focal length is a measure of the concentration or divergence of light in an optical system. It refers to the distance from the optical center of the lens to the focal point of the light when parallel light is incident. It is also the distance from the center of the lens to the imaging plane such as the film or CCD in the camera. An optical system with a short focal length has a better ability to gather light than an optical system with a long focal length. Simply put, the focal length is the distance between the focal point and the vertex of the mirror. 

Example: The focal length of the camera SPEC table indicates that the focal length is 4.3-86.0mm, which means that the focal length of the camera lens can be adjusted from 4.3mm to 86mm, and the lens speed is 20x optical zoom. Adjust the focal length of the camera lens to adjust the clarity of the image.