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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

[AcrySof® Toric (SN60T) intraocular lens implantation: refractive predictibility and aberrometric impact of decentration].

J Fr Ophtalmol. 2013 Apr;36(4):352-61

Authors: Humbert G, Colin J, Touboul D

PURPOSE: To evaluate refractive outcomes of toric intraocular lens (IOL) implantation with a detailed analysis of decentration and its effect on aberrometry.
METHODS: This retrospective study enrolled 19 eyes implanted with SN60T AcrySof(®) Toric (Alcon - USA) IOL's. Spherocylindric correction was studied in depth by the Alpins method, and retro-illumination images were used to analyze the alignment of the IOL with its intended axis. IOL decentration as well as its aberrometric impact were evaluated with a strict, novel protocol.
RESULTS: Three months postoperatively, uncorrected distance visual acuity was greater or equal to 20/40 in 94.74% of cases, postoperative subjective cylinder was less or equal to 0.5 D in 68.42% of eyes, with a mean index of success of 0.24. Mean error of toric IOL alignment was 5.68° (0 to 14). Mean IOL decentration was 0.78 mm (0 to 1.78) with a mean coma and trefoil of 0.18 μ (0.06 to 0.33) and 0.19 μ (0.05 to 0.51), respectively. The larger the IOL decentration, the higher the optical aberrations were.
CONCLUSION: Toric intraocular lens implantation is an effective, safe and predictable method of spherocylindrical correction during cataract surgery, with a refractive accuracy similar to that of LASIK in the treatment of astigmatism in young patients. IOL decentration produces optical aberrations including coma and trefoil, which interfere with visual performance.

PMID: 23332291 [PubMed - indexed for MEDLINE]


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