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{{Unreferenced|date=January 2007}}
[[Image:Focal-length.png|frame|right|The focal point '''F''' and focal length ''f'' of a positive (convex) lens, a negative (concave) lens, a concave mirror, and a convex mirror.]]
In [[optometry]], the '''least distance of distinct vision (LDDV)''' or the '''reference seeing distance (RSD)''' is the closest someone with "normal" vision ([[Snellen chart|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.



==LDDV and lenses==
The LDDV is used to define the magnifying power (''M'') of a lens with [[focal length]] (''f'' in millimeters) when viewed by the naked human eye:

:<math>{M} = \frac{250}{f}.</math>

==LDDV and Snellen charts==
[[Snellen chart]]s refer to someone with 20/20 vision being able to discriminate spatial patterns separated by a [[visual angle]] of 1 [[minute of arc]] (1/60th of a [[degree (angle)|degree]]).  This is the same as someone with 20/20 vision is able to comfortably view a dot of 73 [[micrometre|µm]] at 250&nbsp;mm:

:<math>250 \sin \frac{1}{60} = 0.07272\ \mathrm{mm} = 72.72 \mu\ \mathrm{m}.</math>

==See also==
*[[Optometry]]
*[[Far point]]
*[[Snellen chart]]
*[[Visual perception]]
*[[Visual impairment]]

{{DEFAULTSORT:Least Distance Of Distinct Vision}}
[[Category:Optometry]]


{{Optics-stub}}