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.


 * The LDDV is widely accepted as being 250mm (10 inches).

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:


 * $${M} = \frac{250}{f}.$$

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 (1/60 th 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:


 * $$250 \sin \frac{1}{60} = 0.07272\ \mathrm{mm} = 72.72 \mu\ \mathrm{m}.$$