Least distance of distinct vision
- Least distance of distinct vision
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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 (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: 20ft
LDDV and Snellen charts
Snellen charts 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). This is the same as someone with 20/20 vision is able to comfortably view a dot of 73 µm at 250 mm:
See also
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