visual perception Visual perception is the ability to interpret the surrounding environment through photopic vision (daytime vision), color vision, scotopic vision (night vision), and mesopic vision (twilight vision), using light in the visible spectrum ref ...

, the near point is the closest point at which an object can be placed and still form a focused image on the

retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which the ...

, within the eye's accommodation range. The other limit to the eye's accommodation range is the

far point In visual perception, the far point is the farthest point at which an object can be placed (along the optical axis of the eye) for its image to be focused on the retina within the eye's accommodation. It is sometimes described as the farthest p ...

. A normal eye is considered to have a near point at about for a thirty year old. The near point is highly age dependent (see accommodation). A person with

hyperopia Far-sightedness, also known as long-sightedness, hypermetropia, or hyperopia, is a condition of the eye where distant objects are seen clearly but near objects appear blurred. This blurred effect is due to incoming light being focused behind, i ...

presbyopia Presbyopia is physiological insufficiency of accommodation associated with the aging of the eye that results in progressively worsening ability to focus clearly on close objects. Also known as age-related farsightedness (or age-related long si ...

would have a near point that is farther than normal. Sometimes, near point is given in

diopter A dioptre (British spelling) or diopter (American spelling) is a unit of measurement with dimension of reciprocal length, equivalent to one reciprocal metre, 1 dioptre = 1 m−1. It is normally used to express the optical power of a lens or cur ...

s (see ), which refers to the inverse of the distance. For example a normal eye would have a near point of

\frac = 9\ \text

Vision correction

A person with hyperopia has a near point further than comfortable, (i.e. 25 cm) , and hence is unable to bring an object 25 cm away into sharp focus. A

corrective lens A corrective lens is a lens (i.e. a transmissive optical device) that is typically worn in front of the eye to improve daily vision. The most common use is to treat refractive errors: myopia, hypermetropia, astigmatism, and presbyopia. G ...

can be used to correct hyperopia by imaging an object at distance onto a

virtual image In optics, an ''image'' is defined as the collection of focus points of light rays coming from an object. A '' real image'' is the collection of focus points made by converging rays, while a virtual image is the collection of focus points ma ...

at the patient's near point. From the thin lens formula, the required lens will have

optical power In optics, optical power (also referred to as dioptric power, refractive power, focusing power, or convergence power) is the degree to which a lens, mirror, or other optical system converges or diverges light. It is equal to the reciprocal of the ...

given by

P \approx \frac-\frac

. 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-\frac

. For example, if a person has , then the optical power needed is where one

is the

reciprocal Reciprocal may refer to: In mathematics * Multiplicative inverse, in mathematics, the number 1/''x'', which multiplied by ''x'' gives the product 1, also known as a ''reciprocal'' * Reciprocal polynomial, a polynomial obtained from another pol ...

of one meter.

References

Ophthalmology {{eye-stub