What Is a Diopter?

Eyeglass prescription
Troy Bedinghaus

Question: What is a diopter?

Answer: A diopter is a unit of measurement used to denote the refractive power of vision correction devices. Optometrists measure refractive error in terms of diopters. Your eyeglass or contact lens  prescription is a measurement of your refractive error, or the degree from which you do not have normal vision. Eyeglass prescriptions are written with standardized numbers and abbreviations, making them able to be interpreted worldwide.

Diopter and Prescriptions

Prescriptions are commonly measured in units called diopters. A diopter is usually represented by a capital "D" in the prescription. The numbers you see in the sphere and cylinder columns of an eyeglass prescription tell the optical power of the lenses in diopters. Higher diopter numbers signify that the lens refracts or bends light more than that of lower diopter numbers. A diopter is the reciprocal of the focal length, measured in meters. For example, if a lens has a focal length of 13 meters, it is a 3 diopter lens.

Refractive Errors

Diopters represent the level of correction you need in your eyeglasses or contact lenses to achieve normal vision. The higher your prescription in diopters, the more nearsighted or farsighted you are. Put simply, a prescription for higher diopter correction means you need more power in your lenses to bring vision back to a normal state.

The following are the most common refractive errors:

  • Nearsightedness: Nearsightedness, or myopia, is a condition in which nearby objects are seen clearly, but distant ones are blurred. Nearsightedness can be inherited and is often discovered during childhood.
  • Farsightedness: Farsightedness, or hyperopia (also referred to as hypermetropia), usually causes distant objects to be seen clearly, but close objects to appear blurred. Farsightedness often runs in families. When someone has higher levels of farsightedness, their distance vision may become blurry in addition to their near vision. Many people mistake farsightedness for presbyopia, the refractive error that usually occurs over 40 years of age.
  • Astigmatism: Astigmatism usually occurs when the cornea has an irregular curvature. The cornea is curved more in one direction, causing blurry vision. Astigmatism can cause blurry vision at all distances, and it often occurs along with farsightedness or nearsightedness. Most people have very small amounts of astigmatism. Larger amounts of astigmatism cause distortion in addition blurry vision. Very high amounts of astigmatism sometimes have a difficult time achieve 20/20 vision.
  • Presbyopia: Presbyopia is the normal aging process of the lens of the eye. It is the loss of elasticity of the lens that occurs with aging, causing difficulty focusing at close ranges. Scientists also believe that in addition to the loss of elasticity of the lens, the muscle that makes the lens change focus, called the ciliary body, also begins to not work as well. Presbyopia usually becomes significant after the age of 40-45 years of age but people between 35-40 may exhibit early signs depending on their visual state, work and lifestyle.


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