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. The magnifying power (M) of a lens with focal length (f in millimeters) when viewed by the naked human eye can be calculated as:
M = 250 f . {\displaystyle {M}={\frac {250}{f}}.}
See also Optometry Far point Snellen chart Visual perception Visual impairment
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