Your Prescription Explained

Have you ever wondered what all the numbers mean on your prescription?
Yes? OK,  so just for you we have tried to explain it all as simply as possible in plain English.

Here is a typical Prescription (sometimes written as Rx) it could be written or typed on plain paper, in a table on an opticians own form or on an NHS prescription form. (see examples below)

R +1.00/-1.50 x 85  Add +2.00  Prism 1.5 out
L + 1.25/-1.75 x 75  Add +2.00  Prism 1 up

bizarrely this can also be written as

R -0.50/+1.50 x 175  Add +2.00 Prism 1.5 Out
L -0.50/+1.75 x 165  Add +2.00 Prism 1 Up

Both will give exactly you the same vision.


You may occasionally come across the abbreviations “ OD, OS & OU”, these are Latin abbreviations for Oculus Dexter, Oculus Sinister and Oculus Uterque which simply means Right eye, Left eye and Both eyes.
The measurements are in Dioptres – usually abbreviated as D’ (A Dioptre is the unit of refractive power equal to the reciprocal of the focal length – sorry that’s a bit technical but unfortunately it is the dictionary definition) . In the example above, this person has  1 dioptre in the Right eye and 1¼ in the Left eye. The lower the number the less short sighted or long sighted you are. A ‘+’ preceding the number denotes long sightedness (Hyperopia) and a ‘-‘ short sightedness (Myopia)

Now for the other abbreviations:-

SPH (sphere) is the part of the prescription that is spherical and denotes long or short sightedness as discussed above. If there are no measurements under CYL or AXIS then your eye is equal in all meridians ( often described as football shaped).

CYL (cylinder) this is the measurement of the amount of astigmatism you have, The higher the number the more astigmatism you have. Astigmatism is just a word that describes the shape of you eye (often described as rugby ball shaped, the higher the number the more rugby ball shaped your eye is). It is this correction in your eye that will determine how sharply you see an image. (similar to fine tuning a TV once the channels have been found).

AXIS  This measurement will be between 0 and 180 degrees and denotes the orientation of the astigmatism. Imagine a line drawn through the two points of a rugby ball, this line has to be orientated on the correct axis for you to see clearly. The higher the astigmatism the more accurate this has to be.

ADD  This is the extra magnification needed for near vision to correct presbyopia (another word for problems with near vision as you age). This number is always a positive and usually preceded by a ‘+’ sign. Occasionally you may see ‘INTER ADD’ ( or intermediate add) on your prescription, this is when you need a specific prescription for VDU’s or if you have a longer working distance than a normal reading distance. Again this will be a positive number but will be a lower number than the near vision addition.

PRISM  This is prescribed when the eyes need a little help to correct a muscle balance defect. With this in the lens, the eyes work together properly and any double vision can be eliminated.
Prism is measured in dioptres with increments of 0.25 or ¼ dioptres. You will notice there is also an indication of direction (up, down, in or out) or combinations of directions (i.e. 2in1up). On a hand written prescription prism is sometimes denoted with a triangle preceding the numbers. The direction indicates where the “base” or thickest part of the prism should be.

Two things to remember:-

A/ a spectacle prescription is different from a contact lens one

B/ Once you have been tested, the prescription is yours to keep,you are not obliged to buy spectacles from the optician that provided the examination. It is worth shopping around though if you have problems they may be more difficult to sort out. However here at One2One we have a no quibble satisfaction guarantee – If we are unable to resolve any issues to your satisfaction we will happily give you a full refund.