The normal human eye can see objects clearly that are from 25 cm away to infinitely far away (for example, the distant stars should appear as sharp, well-defined points of light). Myopic (or nearsighted) eyes can see things very well that are close to the eye (often much closer than the normal human-eye near point of 25 cm!), but have difficulty keeping the object focused as it moves further away from the eye. The point at which a myopic eye can no longer focus on the object is called the eye’s far point, and is at a distance we shall denote Dmax. For the myopic eye, Dmax < ¥, the far point of the normal human eye. To see objects clearly that are beyond the myopic eye’s far point, corrective lenses or surgery are required. In this case, the proper corrective lenses must take light from an object that is very far away (essentially infinitely far away: do = ¥) and form a virtual image that is at the myopic eye’s far point, so that Di = Dmax, and di = –Dmax. (Do you see why the image must be virtual?)
As a final note concerning vision defects, we point out that an astigmatism is an eye defect in which light from a point source (such as a star or distant street lamp) is imaged as a line on the retina instead of as a sharp point of light. This results from either the cornea or the lens (or both) not being perfectly spherical. Special lenses are required to correct such eye defects in which the radius of curvature of the cornea or lens is different along two mutually perpendicular axes along the surface of the cornea or lens.