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Decoding your Prescription

Many patients find that when they are given their prescription after an eye test the jargon used means nothing to them. We feel it is about time that we, as opticians, decoded your prescription and explained what the terminology used means.


Here is an example of the type of traditional prescription a you may receive, this prescription will be different to the one you receive if you have requested contact lenses- we will go into this in more detail later on.

Eye Prescription Example Draft

There will be two different sections in your prescription, one for your left eye and one for your right eye, on occassion the Latin abbreviations may be used – OS (oculus sinister) for the left eye and OD (oculus dextrus) for the right eye. Occasionally you may see a notation for OU, which means a prescription for both eyes. These sections will usually have the following headings within them:

SPH (sphere)
A + in the box indicates that you are long sighted (hyperopia), means you find it difficult to see things close to you. A – shows that you are short sighted (myopia), meaning you find it hard to see things in the distance without a prescription. The higher the number in the box, the stronger the prescription lenses will be. The smallest prescription number is 0.25, however it is unlikely that opticians will prescribe glasses for a prescription this low. Glasses will usually be prescribed if the number is 1.0 or higher as this is when the lack of vision may being to affect a patients every day life, especially when driving. These numbers are likely to effect your choice of frames, as the higher the prescription, the greater the curve of the lens needed.

CYL (cylinder)
The amount of astigmatism (visual distortion) that is caused by an irregularly shaped cornea. If this box is empty, this means that there is no astigmatism and your eyes are perfectly spherical therefore causing no distortion. If there is a low number, such as 0.25, it means that your eyes are nearly round but not quite. A higher number, such as 3.00, shows that your eyes are quite oval in shape.

This number is an indicator of the direction of the astigmatism, measured in degrees. This number does not relate to how well you can see but it helps opticians to know what angle is best to position your lenses in the frame to be most effective in your vision correction.

This usually indicates that your eyes do not work well together, for example if there is muscle imbalance between your eyes, prism lenses will provide the correction you need and help prevent double vision or headaches that can occur in some cases.

The base simply tells opticians where to put the prism in your glasses, again to ensure the best vision correction.

In some cases, often for those over the age of 45, there may be a number in a box marked ‘ADD’. This is your reading addition and relates to the amount of additional correction needed to focus at close distances. If a measurement is shown in this section, it means you have different prescriptions for distance and reading and bifocal or varifocal lenses may be needed.

 The numbers shown in these sections are given in dioptres (which can often be abbreviated to ‘D’), which is the unit used to measure the correction, or focusing power, that your eyes require.

Contact Prescription Example Draft
A contact lens prescription will have different to sections to that of a glasses prescription and will often include the following sections:

BC (Base Curve)
This number will determine what type of fit is required for your contact lens to meet the curve of your eye, this will usually be written in millimetres or can sometimes be written with the words: flat, median or steep.

DIA (Diamater)
This number will tell the optician which width of lens will best suit your eye, again usually written in millimetres.

SPH (Sphere)
This number, as before, will state how long or short-sighted your vision is. This number can sometimes be presented under the heading PWR (power).

If you suffer with astigmatism you may find that your prescription has additional sections, including CYL (Cylinder) and AX (Axis) which will show the same measurements as in a glasses prescription.

Where there is a need for multifocal correction (presbyopia), usually to help with reading, there may be the following sections included ADD (Addition) which determines the level of correction your need to be able to see clearly at a close distance, usually between 0.50 and 3.00 although some contact lens brands can refer to this as high medium or low. If you need to wear multifocals or bifocals then your lens correction will be determined by a dominant and non-dominant eye, which is usually marked with a ‘D’ and ‘N’ to express which eye is which.

So why is there the need for two different types of prescription?

While both of your prescriptions are likely to be similar, you need two different ones as they will each measure different things. As a contact lens will come in direct contact with a patients cornea (which is the clear covering over the front surface of the eye) it is important that additional measurements needed for fitting a contact lens are included in this type of prescriptions, these include: the curvature of the cornea and the diameter of the cornea. A contact lens is usually between 2-4mm larger than the diameter of the cornea.

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