Understanding Your SPECTACLE Prescription

Making sense of your prescription after an eye test is a bit like looking at a code written in a foreign language. Optometrists and ophthalmologists, will give you your prescription using certain abbreviations and specified measurements. Here’s a quick guide on how to understand them:


Some professionals simplify things by writing a prescription using LE and RE - left and right eye- however, this is not always the case. The first thing you need to understand are that some abbreviations come from Latin.

OS and OD respectively refer to Oculus Sinister and Oculus Dextrus. Oculus means eye and sinister, left, and dextrus is right. When referring to both eyes, the Latin term is Oculus Uterque, or OU for short. You may have seen this on your optical prescription.

Let's spell things out a little more.

  • LE or OS (Left Eye): Refers to your left eye prescription
  • RE or OD (Right Eye): Refers to your right eye prescription
  • SPH (Sphere of the eye): Shows how long or short sighted you are
  • CYL (Cylinder of the eye): Refers to part of your prescription that is due to the shape of your eye, its astigmatism
  • Dioptres: Are the units of optical power of the lens shown as 'D'
  • ADD: This is your reading addition and relates to the amount of extra or additional correction needed to focus at close distances.


Now that we’ve got that down, let's look closer at the abbreviations SPH and CYL. First, SPH refers to the sphere power of the eye.

Long sightedness (when the image focuses behind the light sensitive retina inside the eye) is usually shown with a plus (+) sign, f the number is negative, it has a minus (-) sign this indicates short sightedness (when the image focuses in front of the light sensitive retina).

Dioptres are the name given to the units of optical power in the lens to correct your vision. Dioptres are also used for lens curvature, both these are determined using a scientific formula for focal length, where the light is going to be focused by the lens. The larger the number the greater your short or long sightedness. For example if you see written RE: SPH -7.25D, this means that your right eye needs 7.25 diopters of myopic (short sighted) correction. Sphere also means that this power is the same in all directions on your eye. The minus power lenses diverge the light pushing the focus back to fall clearly on the retina. Plus power lenses correct hyperopia (long sightedness) and converge the light to bring it forward to a clear focus.

Next CYL, is the correction needed for people with astigmatism, where usually the cornea (the clear window on the front of the eye) is shaped more like a rugby ball than a soccer ball. In one case the ball is perfectly round, in the other case it has one long flat curve and one short steep curve. The result of the rugby all shape is blurred vision, by using a CYL or cylinder the lens can have power in one direction and no power in the other. Look at a soft drink can, it is a cylinder, If you look at it when it is standing upright the sides are straight but when you look at it from above you see a circle, only the curved part of a cylinder lens creates optical power.

There is also a number called the Axis that says where the cylinder power will sit, It is a number between 0 and 180 degrees where 90 is vertical and 180 is horizontal. Another value which may be present on your prescription is prism, which is used to help with the alignment of the eyes and will be shown as BU, BD, BI or BO. This means the base of the prism is pointing up, down, in or out depending on what the patient needs.

Finally, what is an ADD? Around your 40's you will need help with close vision, this is called presbyopia. To focus up close needs more power than to focus far away so the ADD is just that, it adds on some extra help for reading, sewing, playing cards etc. This part of the prescription is important for reading glasses, extended focus and progressive lenses.

Prescriptions for Contact Lenses

As contact lenses rest directly on the eye, they must be properly fitted and assessed by your optometrist. A contact lens prescription will have most of the same measurements as for glasses BUT the powers need to be changed as the lenses sit on the eye and not a centimetre or so in front of the eye. A contact lens prescription may also designate: the diameter of the lens; its curve that will follow the curve of the cornea; and usually the brand name and specific lens type as well as whether they are for daily, weekly or monthly replacement

If you’re still having trouble reading your prescription, speak to your optometrist.

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