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The pupil of the human eye can range in size from 2 mm to over 8 mm to adapt to the environment. The human eye can detect a luminance from 10 −6 cd/m 2, or one millionth (0.000001) of a candela per square meter to 10 8 cd/m 2 or one hundred million (100,000,000) candelas per square meter.
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. [1] 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.
Near visual acuity or near vision is a measure of how clearly a person can see nearby small objects or letters.Visual acuity in general usually refers clarity of distance vision, and is measured using eye charts like Snellen chart, LogMAR chart etc. Near vision is usually measured and recorded using a printed hand-held card containing different sized paragraphs, words, letters or symbols.
Between these extremes is the pupil diameter that is generally best for visual acuity in normal, healthy eyes; this tends to be around 3 or 4 mm. If the optics of the eye were otherwise perfect, theoretically, acuity would be limited by pupil diffraction, which would be a diffraction-limited acuity of 0.4 minutes of arc (minarc) or 6/2.6 acuity.
The normal range is 12–21 mm. Upper normal limit for people of African origin is a little higher, about 23–24 mm. [3] A difference greater than 2 mm between the eyes is significant. In children and teenagers mean exophthalmometric measurements increase with age: Less than 4 years old (13.2 mm), 5–8 years old (14.4 mm), 9–12 years old ...
Distance PD is the separation between the visual axes of the eyes in their primary position, as the subject fixates on an infinitely distant object. [2] Near PD is the separation between the visual axes of the eyes, at the plane of the spectacle lenses, as the subject fixates on a near object at the intended working distance. [3]
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If one looks at a one-centimeter object at a distance of one meter and a two-centimeter object at a distance of two meters, both subtend the same visual angle of about 0.01 rad or 0.57°. Thus they have the same retinal image size R ≈ 0.17 mm {\displaystyle R\approx 0.17{\text{ mm}}} .