How to read your eyeglass prescription | An easy-to-understand description
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How to read your eyeglass prescription

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reading your eyeglass prescription

Regular eye checks are an important part of keeping your eyes healthy and vision clear. If your vision needs to be corrected, your eye care professional (ECP) will give you a prescription for eyeglasses or contact lenses and recommend the degree of the lens that you need to see clearly. Eye prescriptions contain various numbers and letters that can be confusing at first. This article will help you make sense of these numbers and letters and guide you to read your eyeglass prescription.

Dioptres

Dioptres (D) is the unit used to measure the degree or power of the corrective lens required for clear vision. The further away a number is from 0, the more correction or stronger prescription is needed to produce clear vision for distance and reading. Sphere power, cylinder power, and add power are written in decimal form in quarter dioptre increments (0.25D).

OU: an abbreviation for oculus uterque, Latin phrase for both eyes

OD: an abbreviation for oculus dexter, Latin phrase for right eye

OS: an abbreviation for oculus sinister, Latin phrase for left eye

Instead of OD and OS, some ECPs use RE (right eye) and LE (left eye).

Sphere

Sphere (SPH) indicates the amount of lens power prescribed to correct myopia (short-sightedness) or hypermetropia (long-sightedness). A minus sign (-) means that you are short-sighted and require concave lenses to correct your vision. A plus sign (+) means you are long-sighted and require convex lenses to correct your vision.

For example,

-2.00: represents 2 dioptres of short-sightedness

-6.25: represents 6 and ¼ dioptres of short-sightedness

+3.00: represents 3 dioptres of long-sightedness

Cylinder

Cylinder (CYL) indicates the amount of lens power needed to correct astigmatism. CYL always follows the SPH power on an eye prescription. The number in the cylinder column can be negative (minus) or positive (plus). When a prescription for astigmatism is written in a positive or plus format, the ECP will convert it into a minus cylinder format by using a formula.

For example:

-2.00×180: represents 2 dioptres of astigmatism at an axis of 180 degrees

+5.00×90: represents 5 dioptres of astigmatism at an axis of 90 degrees

Axis

Axis is a number between 0 and 180 degrees that indicates the orientation of the astigmatism. For people with astigmatism, there will be 3 numbers in your prescription: Sph / Cyl x Axis. For example, -2.00/-1.00×180 means 2D of short-sightedness, 1D of astigmatism at an axis of 180 degrees. If you don’t have astigmatism, the cylinder and axis boxes will be empty or only one value is written on the prescription. For example, -1.00D indicates the presence of only 1 dioptre of short-sightedness with no astigmatism.

vision with and without astigmatism

Figure 1. Vision of a person with and without astigmatism.

Near addition

Near addition (ADD) indicates the additional lens power needed to correct presbyopia. The value appearing in this section is always a plus power, even if there is no sign to indicate that. ADD is usually the same for both eyes. You may also have an Intermediate ADD, indicating the lens power needed for computer vision or tasks performed at that distance. While an ADD lens power can differ between individuals, ECPs use an age-based table as a guide when doing a refraction test, as seen below:

Age (years) Near addition at 40cm
40 to 45 Less than +1.00D
45 to 50 +1.25 to +1.50D
50 to 55 +2.00D to +2.25D
55 to 60 +2.25D to +2.50D
60 to 65 +2.50D to +2.75D
More than 65 More than +3.00D

Prism

Prism indicates the amount of prismatic power that your eyeglasses need to compensate for a difference in the alignment of your eyes. If present, along with the amount of prism, the direction of the prism is indicated by noting the position of the base.

Here are the four abbreviations used to indicate prism direction:

  • BU=base up
  • BD=base down
  • BI=base in
  • BO=base.

For example, -2.00D with prism 0.50BD means 2 dioptres of short-sightedness and 0.5 or half a dioptre of prism power at a base down orientation.

Inter-pupillary distance

Inter-pupillary distance (PD) is the distance (in millimeters) between both pupils in the eyes. Monocular PD indicates the distance from your pupil to the middle of your nose. Binocular PD indicates the distance from one pupil to the other.

Visual acuity

Visual acuity (VA) indicates how well you can see with the degree of lenses written in the prescription.

Can I use an eyeglasses prescription to buy contact lenses?

The short answer is no. A prescription for eyeglasses does not contain information for a contact lens prescription. Eyeglasses are positioned at a distance from the eyes, while contact lenses sit directly on the surface of your eyes. The distance affects the power of the lens required to produce clear vision. A contact lens prescription must specify the base curve and diameter of the contact lens.

Example of how to read your eyeglass prescription

Compiling all the components mentioned above, here is an example of what an eye prescription looks like:

SPH CYL Axis ADD Prism
OD -3.00 +2.00 1BD
OS -2.00 -1.00 180 +2.00 1BU

In the right eye (OD), the ECP prescribed:

  • -3.00D for the correction of short-sightedness
  • There is no cylinder power or axis, which means that there is no astigmatism, as indicated with a dash symbol.. Some ECPs may choose to leave it empty or write SPH, indicating that there is only sphere power and no cylinder

In the left eye (OS), the ECP prescribed:

  • -2.00D for the correction of short-sightedness
  • -1.00D cylinder for the correction of astigmatism
  • A cylinder power with an axis of 180 degrees

Both eyes are prescribed with:

  • Near addition or ADD of +2.00D for the correction of presbyopia
  • Prism correction of 0.5 prism dioptre in each eye. In the right eye, the prism is base down (BD) and in the left eye, the prism is base up (BU)

We hope that this article has been helpful for you to better understand how to read your eyeglass prescription. This article is only a guide and is not intended to replace the information that you may have already received from your ECP. As always, it is always best to seek advice directly from your ECP if you have any queries about your eye check visit.

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