• Aberrations: ghost images, halos, waves or rainbows caused by imperfections in the curve or lens surface
  • Index of refraction: a ratio used to compare refractive power
  • Plus lens (+): a lens that is thickest at the center; moves the focal point forward
  • Minus lens (-): a lens that is thinnest at the center; moves the focal point backward
  • Focal point: a spot in space where refracted light meets; may be actual (plus lens) or assumed (minus lens)
  • Pupillary center: the point on a lens directly in front of the pupil
  • Astigmatism: a condition caused by a distortion in the cornea that creates an additional lens power

How Your Eye Focuses

On the back of your eye is a complex layer of cells known as the retina. The retina reacts to light and conveys that information to the brain. The brain, in turn, translates all that activity into an image. Because the eye is a sphere, the surface of the retina is curved.

When you look at something, three things must happen:

  • The image must be reduced in size to fit onto the retina.
  • The scattered light must come together -- that is, it must focus -- at the surface of the retina.
  • The image must be curved to match the curve of the retina.

To do all that, the eye has a lens between the retina and the pupil (the "peep hole" in the center of your eye that allows light into the back of the eye) and a transparent covering, or cornea (the front window). The lens, which would be classified a "plus" lens because it is thickest toward the center, and the cornea work together to focus the image onto the retina. (For more information on how the eye functions, see How Vision Works.)