Least distance of distinct vision of the device in metres. A convex lens and a concave mirror have positive focal lengths, and hence their strength in Dioptres is also positive. If the lens is concave or the mirror is convex, the focal length, and hence the power in Dioptres, is negative. Least distance of distinct vision In optometry, the least distance of distinct vision (LDDV) or the reference seeing distance (RSD) is the closest someone with "normal" vision (20/20 vision) can comfortably look at something. In other words, LDDV is the minimum comfortable distance between the naked human eye and a visible object. magnifying power
Telecentric lens Telecentric lens A telecentric lens is a compound lens that has its entrance or exit pupil at infinity; in the prior case, this produces an orthographic view of the subject. This means that the chief rays (oblique rays that pass through the center of the aperture stop) are parallel to the optical axis in front of or behind the system, respectively. The simplest way to make a lens telecentric is to put the aperture stop at one of the lens's focal points. An entrance pupil at infinity makes the lens object-space telecentric. Such lenses are used in machine vision systems