Notes:

As you might imagine, it is quite a challenge for a growing eye in a new-born infant to mature into a well-focused optical system which is perfectly adjusted to produce clear retinal images of objects located billions of miles away, like the stars in the sky, and a fraction of a second later to focus on an object located only inches away, like the stars in your lover's eyes. So it is not too surprising that sometimes the eye turns out not quite right. That is why God gave us optometrists! - to figure out what sort of auxiliary lenses are needed, such as contact lenses, spectacle lenses, or artificial intra-ocular lenses, to produce clear retinal images in our eyes.

There are many different reasons why the retinal image may be less than perfect, but they all fall into 4 basic categories. By far the most common cause of imperfect retinal images is simply that they are a bit out of focus. The focusing mechanims of the eye consists of an elastic lens, suspended inside the eye by an elaborate system of very tiny ligaments that act like guy-wires, which are connected to a muscle indide the eye. When a person looks at a distant object, this muscle relaxes into an elongated posture which puts tension on the suspension ligaments, which in turn pulls the lens into a flattened posture. Since the optical power of a lens is derived from its curvature, a flat lens has little optical power and so the eye is focused on distant objects.

To focus on something near, the muscle contracts into a tight little ball, which makes the ligaments go slack, and the elastic lens returns to a more natural spherical shape which has greater optical power.

One of the complications of old age is that the lens becomes stiff and won't change shape in response to the changing tension on the suspensory ligaments. This condition is called "presbyopia" by optometrists. Unlike many other complications of old age, presbyopia is easily compensated by reading glasses.