When you go to get an eye test the optometrist will write you a prescription but they may not explain what it actually means. Here is a simple guide to help you understand it.
These are some fairly typical prescription formats. Click on the image to enlarge.
Sph (sphere) represents how long or short sighted you are. This can be positive (+) for long sight or negative (-) for short sight. The higher the number, the stronger your lens.
Cyl (cylinder) represents the amount of astigmatism present. Astigmatism distorts both your near and far vision and is caused by your eye being shaped more like a rugby ball than a football.
Axis indicates the direction of the part of the lens that corrects your astigmatism.
Prism – this bends the light without altering its focus. Prisms are used to help people whose eyes do not work perfectly together (i.e. they turn in too much, or turn out too much), which may cause eyestrain or double vision.
VA (visual acuity) refers to how clearly you can see shapes and detail from a set distance away (six metres in the UK), with your glasses on if you wear them. It is shown as a fraction and the higher the number, the better your visual acuity. For example, if you have a VA of 6/6, you can see really clearly and if your VA is 1/6, it means that you need to be a metre away from an object to see it as clearly as people can normally see from six metres away.
Add (or sometimes just reading or near) refers to the additional magnifying power needed in the bottom part of a multi-focal lens for reading and other close work. This is usually only needed for patients over 45, as focusing ability declines as we get older (presbyopia).
(Information provided by The College of Optometrists)