Interpreting a prescription is not easy - please don't hesitate to contact us if you need any help.  Our optical care advisors, optometrists or surgeons will be able to talk you through what each of the numbers means and how we would expect your prescription to change and effect your vision in years to come.

A prescription usually consists of three (sometimes four) numbers that look like this:
Right Eye - Sphere -2.00 /Cyl -1.00 / Axis 90 (‘+2.00 add’)

The first number is the Sphere.

The higher this number is, the worse your unaided vision (your vision without spectacles or contact lenses). If it’s negative (e.g. -2.00) you are short sighted, while a positive number (e.g. +2.00) indicates long-sightedness.

The second number is the cylinder or cyl.

Not everyone has astigmatism, so you may not have a number in the "cyl" box, but most do. Astigmatism is not a disease, it simply means that in order to focus light into the eye, two different powers are required. These powers are usually positioned perpendicular to each other. This "cyl" number indicates the amount of difference between the two powers.

Most commonly astigmatism is caused by the front surface of the eye, the cornea, which is toroidal in shape and often described as having a surface curvature similar to a rugby ball rather than football shaped spherical surface.  In astigmatism the surface of the eye has two principal curvatures usually perpendicular to each other, one steeper curve, and one flatter curve.

The "Cyl" can have a "–" or a "+" in-front of it, but the amount of astigmatism is more important. The higher this number is, the greater the difference in curvature between the flatter and steeper meridian (toricity).

Astigmatism is very common and can be treated effectively by laser eye surgery along with long and short-sightedness.

The third number is the axis of the astigmatism

You will only have an axis if you have some degree of astigmatism. The axis represents the direction of either the steeper or the flatter meridian. If the cyl has a plus in front of it, the axis represents the direction of greater or steeper curvature, if the cyl has a minus in front of it, then the axis represents the direction of the flatter curvature. A common cause of confusion is that a prescription can be written with a plus cyl or with a minus cyl. The prescription is actually the same, although the numbers on the prescription will differ greatly. Its a little like writing the same words in a different language: The language the optician uses depends on how they were trained and which university they went to.

Don't spend too long on this, but an example is given below.


Example

Sphere -0.50, Cyl -1.50, axis 180, means one meridian is -0.50 short sighted, the difference between the two powers is -1.50, so the second meridian has a total power of -2.00 short sighted, and the direction of the flatter curve is 180 degrees = horizontal (the rugby ball is is lying flat).

or

Sphere -2.00, Cyl +1.50, axis 90, is exactly the same prescription as above although the numbers are totally different. If we write it long hand, it means one meridian is -2.00 short sighted, the difference between the two meridians is +1.50, this means the second meridian is -0.50 short sighted.  The direction of the steepest curve is 90 degrees = vertical. With a little thought you can see that the two prescriptions are describing the same power distribution (the rugby ball is still lying flat on the ground).


A fourth number may be the Add or Addition

Sometimes, there will be an "Add" or "Addition" number. This indicates Presbyopia, or an ageing of the eyes. It represents an additional magnification that is required to help with reading or close focussing, and indicates that the natural lens within the eye is loosing its ability to change its shape to focus on near objects.

Now you should know if you are long-sighted (hyperopic), or short-sighted (Myopic), this may or may not be combined with astigmatism, and if you have an addition you are most likely to have the odd grey hair also (or you will soon).

Just to confuse things a little further, very rarely, a prescription will contain a "prism". prisms are occasionally used in spectacles to help the two eyes work together more comfortably.

As above - please don't hesitate to contact us if you need any help, our optical care advisors, optometrists or surgeons will be able to talk you through what each of the numbers means and how we would expect your prescription to change and effect your vision in years to come.