Vision correction
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in normal vision (emmetropia), both distant and near objects are focused correctly on the retina, through the process of accomodation
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near-sightedness (myopia) occurs when the lens is too strong for the length of the eyeball i.e., distant objects are focussed in front of the retina, but near objects are focussed correctly
- myopia is corrected with the use of a concave lens, which decreases the convergence produced by the cornea and lens
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far-sightedness (hyperopia) occurs when the lens is too weak for the length of the eyeball i.e., distant objects are focussed correctly due to accomodation, but near objects are focussed behind the retina
- hyperopia is corrected through the use of a convex lens which supplements the convergence produced by the cornea and lens
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astigmatism occurs when irregularities in either the cornea or lens cause light to be bent erratically
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presbyopia is a decrease in the ability to focus on near objects that occurs with age
- presbyopia is thought to be caused by a decrease in the elasticity of the lens, which decreases the ability to accomodate