Near point

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.

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.

A normal eye is considered to have a near point at 25 cm. A person with hyperopia or presbyopia would have a near point further than 25 cm away.

Sometimes, near point is also given in diopters (see Presbyopia § Mechanism), which refers to the inverse of the distance stated above (e.g. a normal eye would have a near point of )

Vision correction

A person with hyperopia has 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[1]

.

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:

.

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.

gollark: *I* should be the owner.
gollark: > it should now be clear that he should not be ownerWhy?
gollark: Unfortunately, autobotrobot was moved back out of the "esobot" section.
gollark: Great, they reversed my permission override.
gollark: I was just abusing my admin powers.

References
  1. "Vision Correction | Physics". courses.lumenlearning.com. Retrieved 2019-12-05.
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.