Your eye prescription explained

Posted: May 26 2018

Your eye prescription explained

Have you ever looked at your prescription and wondered what all the numbers, signs and words mean on it? Read our guide below to make sense of your prescription.

When you look at your prescription you will notice the column on the left hand side has a row for ‘OD’ and a row for ‘OS’. These correspond to your right and left eye, they are abbreviations of the Latin terms ‘oculus dexter’ and ‘oculus sinister’. If there is a column labelled ‘OU’ that is an abbreviation of ‘oculus uterque’ which means both eyes.

When you look at the rows the first heading will be ‘sphere’. The sphere is measured in dioptres and indicates the lens power required to correct your vision. A positive sign (+) in this box means you’re long-sighted and struggle to see things close to you without corrective lenses. A negative sign (-) in the box means you are short-sighted and struggle to see objects in the distance without assistance. The higher this number, the stronger the prescriptive lens to correct your vision needs to be, for example, a correction of +0.25 is small compared to one of +6.00.

Next you have the ‘cylinder’. This is the lens power required to correct any astigmatism. Astigmatism is a visual distortion that is caused by your cornea being an irregular shape. If there is no information in this box it means your eyes are spherical and you don’t have astigmatism. A low number will mean they are not quite round and you have a small amount of astigmatism. A high number, such as +/- 4.00, will mean the corneas are more oval and you have a high degree of astigmatism.

prescription chart

The ‘axis’ is the next box on your prescription. This is the direction of any astigmatism and this is measured in degrees, between 1ᵒ and 180ᵒ. This is where the cylinder power is applied to oppose the effect of the oval shaped cornea.

The next box is the ‘prism’. This is measured in prism dioptres and is an indication of whether your eyes work well as a pair or not. The prism lenses provide the correction you need for eye alignment problems and will prevent you developing headaches or double vision. It is indicated as either a decimal or fraction.

Then there is the ‘base’. This is to tell the lab where to put the prism in your glasses. There are four main abbreviations used for the prism direction, such as ‘BU’ which stands for ‘base up’. There is ‘BD’ for ‘base down’, ‘BI’ for ‘base in’ and ‘BO’ for ‘base out’. Base in and base out relate to the position of the prism in relation to the wearer’s nose, e.g. base in means towards the nose and base out means towards the ear.

Finally, there is a section labelled ‘Add’. You may only see a number here if you are 45 or over as this relates to presbyopia. This is the corrective power needed for reading and to see objects clearly at close distance on top of what is required already to see in the distance.

With technology constantly evolving there are surgical options that enable your prescription to be corrected as well as any astigmatism. Get rid of the numbers on your prescription and come in for an initial consultation today to see what options are available to you.

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