Prescriptions for glasses have two main components: shape
The shape of a lens determines
the type of correction.
- Concave, or minus, spherical lenses are thicker
at the sides than in the middle to correct
- Convex, or plus,
lenses are thicker in the middle than at the sides to correct
farsightedness (hyperopia) or
- Cylindrical (toric) lenses are
curved more in one direction than another to make up for irregularities in the
cornea that cause
- Convex lenses or bifocals refocus the image on the retina when
people with presbyopia lose the ability to focus on close objects
around age 40.
The power of a lens determines the
amount of correction. It is specified in
diopters. The higher the number of diopters, the more
vision correction the lens provides.
Adam Husney, MD - Family Medicine
Christopher J. Rudnisky, MD, MPH, FRCSC - Ophthalmology
October 16, 2012
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