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vision defects 
Images do not form on the retina, thus resulting in blurry vision; such defects are corrected by eyeglasses, contact lenses or even surgery.
astigmatism
Usually caused by a curvature of the cornea, it is manifested by blurred vision when viewing both near and far objects, depending on various axes.
focus
Point where light rays converge to form an image; the brain interprets the retina’s upside-down image as right-side-up.
toric lens
Has various powers depending on the rays’ axes of convergence; it is used to offset the visual distortion caused by the cornea.
