Distance between a lens and the image.
The linear distance between a point of reference, usually a principal point, of an optical system and the corresponding primary or secondary focal point.
In optics, the distance from the lens to the point of focus of light rays passing through the lens.
In the realm of optics, there exists a fundamental characteristic shared by all lenses. This property is referred to as the focal length, denoting the distance from the lens at which it will produce an image of an object located at an infinite distance.