Light from distant objects enters the
eye as parallel rays. Light from near objects enters the eyes
as divergent rays. When light rays pass from one transparent
medium (air) into another (the eye’s cornea and lens),
the rays typically shift slightly in direction, a phenomenon
know as refraction. The normal eye
bends the direction of incoming light rays to focus an image
precisely upon the light-sensitive retina. If the focused
image is sharp, then no corrective lenses are needed.
If, however, the focus is at a point other than upon the retina,
a refractive error is said to exist and the resulting image
on the retina is blurred. If the eye’s refracting surfaces
are spherical—i.e., perfectly round like a basketball—the
focus, though perhaps displaced, still converges to a single
point. Spherical refractive errors are the result of either
too much or too little focus. If the focused image point is
in front of the retina—i.e., too much focus—the
error in refraction is called myopia (near-sightedness).
If the image focus would be behind the retina—i.e.,
too little focus—the condition is called hyperopia
(far-sightedness).
Where a non-spherical refractive error is present, the image
focused on the retina is distorted, with no single sharp focal
point at any location. This condition is called astigmatism.
Astigmatism results when the refractive surfaces are “out
of round”, much like the surface of a football or the
back of a teaspoon. Astigmatism may exist alone or in combination
with either myopia or hyperopia.
In each of these refractive errors, the individual experiences
blurred distance vision varying in degree according to the
magnitude of the error. Most typically, distance refractive
errors are managed with single vision corrective
lenses—to correct for the refractive error.
Corrective lenses may take the form of either spectacles or
contacts.
Presbyopia yet another refractive
error which usually arises in middle age, specifically affects
the vision of objects within arms’ reach. Since light
waves from near objects at this range are divergent as they
enter the eye, additional focusing by the eye is necessary
to produce a clear image upon the retina. Presbyopia impairs
the eye’s ability to adjust its focus from distance
viewing to near viewing. A single vision “reading”
correction for near vision is the most common remedy, Where
both distance and near vision need correction, multi-focal
lenses are often the solution.
The accompanying Glossary
of Optical Terminology is intended to familiarize
you further with a variety of spectacle lens designs and features
so you may be a more informed consumer of such products.
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