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Young adults with 20/20 vision can often focus at 4 to 6 inches. At 6 inches, they can see detail at 600 pixels per inch or 300 line pairs per inch. As you age, your lenses become less resilient and the focus distance increases. At about 50 years, the average minimum focus distance is about a foot corresponding to 150 line pairs per inch or 300 pixels per inch. These numbers reflect good light and high contrast scenes. Generally, under more typical conditions, people don't see this well. --.
The distance between the eye-lens and the retina is about 20 mm. When an object is very far away from the eye, the image is located essentially at the focal point. Therefore the focal length of the cornea and the eye-lens should be about 20 mm when the muscles of the eye are relaxed.Since the strength of a lens is the reciprocal of its focal length in meters, the strength of the cornea and the eye-lens in this situation is about 1/0.020 = 50 diopters.he distance between the eye-lens and the retina is about 20 mm. When an object is very far away from the eye, the image is located essentially at the focal point. Therefore the focal length of the cornea and the eye-lens should be about 20 mm when the muscles of the eye are relaxed.