Understanding your prescription for glasses
When you look at your prescription for glasses, you will see numbers listed under the headings of L for left eye and R for right eye. Sometimes the Latin abbreviations are used -- OS (oculus sinister) for the left eye and OD (oculus dextrus) for the right eye. Occasionally you will see a notation for OU, which means something involving both eyes.
In general, the further away from zero the number on your prescription, the poorer your eyesight and the more vision correction you need. A plus sign in front of the number means you are long-sighted, also called hyperopia (difficulty focusing on close objects), and a minus sign means you are short-sighted, also called myopia (difficulty focusing on distant objects).
These numbers represent dioptres, the unit used to measure the correction, or focusing power, of the lens your eye requires. Dioptre is often abbreviated as D. For example, if your prescription says -1.00, you have one dioptre of short-sightedness. This is a fairly mild degree of myopia. If you are -4.25, that means you have 4 and 1/4 dioptres of myopia. This is more short-sighted than -1.00, and requires stronger lenses. Similarly, +1.00 would be a small degree of long-sightedness and +5 would be greater.
For people who have astigmatism (when the eye is not totally spherical), there will be three numbers in the prescription. The general form for writing these numbers is Sph x Cyl x Axis.
The Sph refers to the "spherical" portion of the prescription, which is the degree of short-sightedness or long-sightedness discussed above.
The Cyl refers to the "cylinder" or degree of astigmatism present, and can be a negative or a positive number. It measures in dioptres the degree of astigmatism. The bigger this number, the more astigmatism there is.
The Axis is a number anywhere between 0 and 180 degrees. It reveals the orientation of the astigmatism. It is not enough to specify how much astigmatism there is; you have to know where the difference in curvature is taking place.
Here are two examples of what prescriptions for eyes with astigmatism could look like:
-2.00 +1.50 x 180
+3.50 +3.00 x 45
The first prescription means that the person has 2 dioptres of short-sightedness, with 1.5 dioptres of astigmatism and an axis of 180 degrees.
The second prescription means that the person has 3.5 dioptres of long-sightedness, 3 dioptres of astigmatism and an axis of 45 degrees.
There will also be a column headed ‘prism’. This indicates the amount of correction that may be needed in some people to align the eyes so that they are looking straight and working well together. A prism is a lens that bends the path of light without altering its focus.
Your optometrist is obliged to give you a copy of your prescription at the end of every eye test.