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.
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.
Point where light rays converge to form an image; the brain interprets the retina’s upside-down image as right-side-up.
Has various powers depending on the rays’ axes of convergence; it is used to offset the visual distortion caused by the cornea.