homeaccount_circle
Human EyeComments

Human Eye

Term Units Description
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 Myopia (nearsightedness) occurs when the focal point is in front of the retina. A correcting lens is a diverging lens.
Hyperopia Hyperopia (farsightedness) occurs when the near point is greater than 25 cm from the eye. A correcting lens is a converging lens.

Corrective Lenses

Term Units Description
Power 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.
Vision Description
Farsighted Corrective Power = (Actual N.P.)-1 - (Normal N.P.)-1 The normal eye near point is .25 meters.
Nearsighted Corrective Power = - (Actual Far Point)-1 The normal eye far point is ∞ and 1/∞ is zero.
Eye-Lens Distance dx 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