The optical structure of an eye is similar to that of a camera. The light rays from an object refract at the cornea and then pass through the crystalline lens (a lens with a variable focal length), and produce an image on the retina. The retina plays the same role as film does in a camera. The main peculiarity of the eye is its ability to change its optical properties depending on the position of an object. This quality is called accommodation.
The extremes of the range over which distinct vision of the normal eye is possible are known as the far point and the near point of the eye.
The distance at which the convenience of viewing the object?s details is the greatest is known as the distance of normal vision.
The far point of a normal eye is infinity. The distance of normal vision of an average eye is considered to be d0=25 cm. The position of the near point depends on the amount by which the ciliary muscle can increase the curvature of the crystalline lens.
In the myopic (near-sighted) eye, rays from an infinitely distant object are focused in front of the retina. In the hyperopic (far-sighted) eye, the image of an infinitely distant object is projected behind the retina.
The distance of normal vision for a normal eye is greater than that of the myopic eye and less than that of the hyperopic eye.
Defects in eyesight can be corrected by the use of eyeglasses. The hyperopic eyes need positive (converging) lenses, and the myopic eyes need negative (diverging) lenses.