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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. --.
As s1 is decreased, s2 must be increased. For example, consider a normal lens for a 35 mm camera with a focal length of f = 50 mm. To focus a distant object (s1 â â), the rear nodal point of the lens must be located a distance s2 = 50 mm from the image plane. To focus an object 1 m away (s1 = 1,000 mm), the lens must be moved 2.6 mm farther away from the image plane, to s2 = 52.6 mm.