Near point

This article is about a measure of an individual's ability to see close (near) objects as clear and in focus. For the near point of convergence, see Vergence § Convergence.

In visual perception, the near point is the closest point at which an object can be placed and still form a focused image on the retina, within the eye's accommodation range. The other limit to the eye's accommodation range is the far point.

A normal eye is considered to have a near point at about 11 cm (4.3 in) for a thirty year old. The near point is highly age dependent (see accommodation). A person with hyperopia or presbyopia would have a near point that is farther than normal.

Sometimes, near point is given in diopters (see Presbyopia § Mechanism), which refers to the inverse of the distance. For example a normal eye would have a near point of ${\frac {1}{11\ {\text{cm}}}}=9\ {\text{diopters}}$.

Vision correction

A person with hyperopia has a near point further than comfortable, (i.e. 25 cm) NP > 25 cm, and hence is unable to bring an object 25 cm away into sharp focus. A corrective lens can be used to correct hyperopia by imaging an object at distance D = 25 cm onto a virtual image at the patient's near point. From the thin lens formula, the required lens will have optical power P given by

$P\approx {\frac {1}{D}}-{\frac {1}{\mathit {NP}}}$.

The calculation can be further improved by taking into account the distance between the spectacle lens and the human eye, which is usually about 1.5 cm:

$P={\frac {1}{D-0.015\;{\text{m}}}}-{\frac {1}{{\mathit {NP}}-0.015\;{\text{m}}}}$.

For example, if a person has NP = 1 m, then the optical power needed is P = +3.24 diopters where one diopter is the reciprocal of one meter.