The Complete Handbook of Contact Lenses

Chapter 1: Understanding Contact Lenses

A contact lens is a thin, curved optical disc—typically between 0.06 and 0.20 millimeters thick—designed to sit directly on the cornea (the transparent front surface of the eye) to refract light and correct refractive errors that prevent clear vision. Unlike eyeglasses, which sit 12 to 14 millimeters in front of the eye, contact lenses work because they move with the eye, maintain a consistent position relative to the pupil, and interact directly with the eye's tear film. This fundamental difference in placement and function determines almost everything about how contact lenses perform, how they're fitted, how they're maintained, and why they're chosen in specific situations.

To understand why contact lenses exist and how they work, you need a basic grasp of how the eye focuses light. The human eye is essentially a camera. Light enters through the cornea and the lens, both of which bend (refract) light rays so they converge on the retina at the back of the eye. The retina converts that focused image into electrical signals that the optic nerve sends to the brain. If the cornea or natural lens focuses light either in front of or behind the retina—rather than exactly on it—the image lands out of focus, and vision becomes blurred. This is a refractive error.

There are three primary refractive errors. Myopia (nearsightedness) occurs when the cornea or eye is too curved, so light focuses in front of the retina; distant objects appear blurred while near objects remain clear.