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Human Eye

By Levi Clancy for Student Reader on

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Near Point (NP, N)

meters (m)

Closest distance the eye can focus clearly. A normal eye has a near point of 25 centimeters. For a relaxed eye the near point is ∞.

Far Point (FP)

meters (m)

Farthest distance the eye can focus clearly. A normal eye has a far point of ∞.


Myopia (nearsightedness) occurs when the focal point is in front of the retina. A correcting lens is a diverging lens.


Hyperopia (farsightedness) occurs when the near point is greater than 25 cm from the eye. A correcting lens is a converging lens.

Corrective Lenses



Diopter (D)

The power of a corrective lens is F-1 and thus has units m-1 (diopters). A negative Power means nearsightedness, while a positive Power means farsightedness.



Corrective Power = (Actual N.P.)-1 - (Normal N.P.)-1
The normal eye near point is .25 meters.


Corrective Power = - (Actual Far Point)-1
The normal eye far point is ∞ and 1/∞ is zero.

Eye-Lens Distance


This is the distance from eye to corrective lens. Many assignments disregard the distance between the eye and corrective eyeglass lenses, and assume it to be zero. If this is not disregarded, then just subtract this distance from the near/far points. The distance between the eye and corrective contact lenses is of course zero.

Magnifying Glasses

Magnification (M)

M = θ' / θ
M = 1 + ( di / F ) = NP / do

Using magnifying glass: object placed much closer to the eye, and image at near point.

Magnifying glass Magnifying Power M = θ' / θ = N.P. / do
θ = h / N.P.
θ' = h / do