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How To Read Eyeglass Or Contact Lens Prescription

Detailed explanation of the meaning of an eyeglass or contact lens prescription. The fist step to considering Lasik.

Image of eye chart with glasses.  
Knowing your eyeglass or contact lens prescription is an important part of knowing if Lasik is right for you.  

Reading your eyeglass or contact lens prescription is often the first step to understanding if conventional or custom wavefront Lasik, Bladeless Lasik, PRK, LASEK, Epi-Lasik, RLE, or any refractive surgery procedure is appropriate for you. You should understand your glasses and contact lens prescription even before you select a Lasik Surgeon.

Bending Light

Refractive error, or the degree from which you do not have normal vision, is commonly measured in diopters and indicates the amount light bends within your eye to be focused on the retina and be "seen". A diopter is often represented by a capital "D" in a prescription. No refractive error is referred to as "plano", often represented as "pl". The greater the refractive error, the larger the number for both sphere, representing the amount of myopia or hyperopia, and and cylinder, representing astigmatism. This known as a spherocylinder or spherocylindrical prescription.

Spherocylindrical Prescription

A typical spherocylindrical prescription would look like:

  sphere cylinder axis
OD: -2.75 -1.25 x15


-0.75 x85

OD is an abbreviation for the Latin oculus dexter, meaning right eye. OS is an abbreviation for the Latin oculus sinister, meaning left eye. The first number after the determination of which eye is the sphere. A negative number indicates myopia. A positive number indicates hyperopia. The second number in this prescription is the cylinder (astigmatism), and the third number is the axis of the cylinder component. The axis of the astigmatism does not relate to the amount of cylinder, just the location of the irregularity. If the patient has no cylinder, then the last two columns may remain blank, or "DS" for "diopter sphere" may be used.

This prescription shows that the patient has 2.75 diopters of myopia with 1.25 diopters of astigmatism at an angle of 15 degrees in the right eye, and the left eye is plano with 0.75 diopters of astigmatism at an angle of 85 degrees.


  sphere cylinder axis
OD: -2.75 -1.25 x15


-0.75 x85

The above prescription shows that the patient has 2.75 diopters of myopia with 1.25 diopters of astigmatism at an angle of 15 degrees in the right eye, and the left eye is plano with 0.75 diopters of astigmatism at an angle of 85 degrees.

Minus Cylinder or Plus Cylinder

Spectacle prescriptions can be written in two value sets, minus cylinder or plus cylinder, which are mutually exclusive of each other but provide the same information. As a rule, ophthalmologists write scripts in minus cylinder whereas optometrists write scripts in plus cylinder. Why these two professions cannot get together and decide on a common method of reporting refractive error is impossible to explain, but if you have ever had competing siblings in your family, you may get an idea why this has not yet occurred.

To convert a minus cylinder form prescription into plus cylinder, or to convert the plus cylinder form into minus cylinder, do the following:

  1. Add the sphere and cylinder powers together; this becomes the new sphere power.
  2. Change the sign of the cylinder power, from minus (–) to plus (+) or from plus (+) to minus (–).
  3. Change the axis value by 90?, remembering that the axis must be a number from 1 to 180.

The following lens prescriptions, therefore, are equivalent and interchangeable:

These two prescriptions are exactly the same
presented in both minus cylinder and plus cylinder form.
  sphere cylinder axis   sphere cylinder axis
OD: -2.75 -1.25 x15 OD: -4.00 +1.25 x105


-0.75 x85 OS:


+0.75 x175
minus cylinder form plus cylinder form

Spherical Equivalent

The spherical equivalent power of a lens prescription is the average of the dioptric powers in all meridians of a lens. To obtain this value, add half of the cylinder power to the sphere power. In other words, do the following:

  1. Divide the cylinder power by 2.
  2. Add this value to the sphere power; the result is the equivalent sphere power of the lens.

For the glasses prescription...

  sphere cylinder axis
OD: -2.75 -1.25 x15


-0.75 x85
minus cylinder form

...the equivalent sphere powers of each lens would be calculated as follows:

OD: –2.75 D + (–1.25 D ÷2) = 2.75 D + 0.625 D = –3.375 D
OS: 0.00 D + (–0.75 D ÷2) = 0.00 D + 0.375 D = –0.375 D

Eye Prescription Terms


Table of Eye Prescription Terms
  Right Eye
  Left Eye
  Inside curve of your contact lens (8.0, 8.1, 8.2, etc.)
  Size of your contact lens (13.8, 14.0, 14.2, etc.)
  [Pow] Strength of your corrective lens (-1.00, -2.75, +2.25, etc.)
  [Cyl] Strength of your astigmatism (-0.75, -1.00, -1.25, etc.)
  Orientation of your astigmatism in degrees (170, 160, 090, etc.)
  No refractive error
  No refractive error
  Bifocal plus power for near distance (+1.00, +2.00, etc)

Looking For Best Lasik Surgeon?

If you are ready to choose a doctor to be evaluated for conventional or custom wavefront Lasik, Bladeless Lasik, PRK, or any refractive surgery procedure, we recommend you consider a doctor who has been evaluated and certified by the USAEyes nonprofit organization. Locate a USAEyes Evaluated & Certified Lasik Doctor.

Personalized Answers

If this article did not fully answer your questions, use our free Ask Lasik Expert patient forum.

Recent Lasik Medical Journal Articles...

Related Articles

Corneal coupling of astigmatism applied to incisional and ablative surgery.

J Cataract Refract Surg. 2014 Nov;40(11):1813-27

Authors: Alpins N, Ong JK, Stamatelatos G

PURPOSE: To redefine measures of corneal coupling for use with incisional and ablation procedures for astigmatism.
SETTING: Private clinics, Melbourne, Victoria, Australia.
DESIGN: Retrospective nonrandomized study.
METHODS: The measures known as the coupling ratio (CR) and coupling constant (CC) were redefined to ensure validity in most cases of incisional procedures and laser vision correction procedures. In addition, a new measure--the coupling adjustment (CAdj)--was developed to quantify the amount of spherical adjustment that must be applied to compensate for coupling that occurs as a result of astigmatism treatment. These quantitative measures of coupling were applied to retrospective data to show their applicability.
RESULTS: Pure myopic, compound myopic, and compound hyperopic astigmatism excimer laser treatments showed a CR close to zero, a CC close to 0.5, and a CAdj close to zero. Incision LRIs showed a CR close to 1.0 and a CC close to zero. In all cases, the coupling measures were consistent for treatments with a larger astigmatic component (>1.0 diopter) but variable when the astigmatic component of the treatment was smaller.
CONCLUSIONS: The revised definitions of CR and CC can be used with incisional and ablative surgery. Incorporating the CAdj into the planning of spherocylindrical treatments allows one to factor in the effect of the astigmatic treatment on the spherical component and thus to more accurately target the desired spherical equivalent.
FINANCIAL DISCLOSURE: Dr. Alpins and Mr. Stamatelatos have a financial interest in the Assort software program. Dr. Ong is an employee of Assort.

PMID: 25442882 [PubMed - indexed for MEDLINE]


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