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Reference.org
Least distance of distinct vision
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<p>In <a href="/facts/Optometry/7QlrKXm2">optometry</a>, the least distance of distinct vision (LDDV) or the reference seeing distance (RSD) is the closest someone with "normal" vision (<a href="/facts/Snellen_chart/cR0DUR4d">20/20 vision</a>) can comfortably look at something. In other words, LDDV is the minimum comfortable distance between the naked human eye and a visible object. </p><p>The magnifying power (<i>M</i>) of a lens with <a href="/facts/Focal_length/brGIUkEY">focal length</a> (<i>f</i> in millimeters) when viewed by the naked human eye can be calculated as: </p> M = 250 f . {\displaystyle {M}={\frac {250}{f}}.}