What does SLR camera lens 18-55mm mean? Be specific.

18-55mm SLR camera lens means focal length.

Focal length is a measure of light convergence or divergence in an optical system, which refers to the distance from the optical center of the lens to the focal point of light convergence when parallel light is incident. In a camera, it is the distance from the center of the lens to the imaging plane (such as the back plate or CCD). An optical system with a short focal length has a better ability to collect light than an optical system with a long focal length. Simply put, the focal length is the distance from the focal point to the center of the mirror. And 18~55mm is the focal length of the zoom lens, which can be adjusted from 18mm to 55mm.

The concept of focal length in photography

The focal length is formed by the optical lens of the camera lens. A group of lenses with curvature (convex or concave) on both sides or one side are installed in the metal tube of a camera or projector to form a comprehensive lens. The light emitted from different parts of the object, after passing through the lens, is focused on a point on the negative, so that the image has a clear outline and real texture. This point is called the focus. The focal length is the distance from the point in the lens of the lens to the point where the light can be clearly focused.