An eye test uses several measurements to describe your eyesight & the strength of any vision correction you'll need. Here you'll find an explanation of the most common terms used on your prescription

Changes & differences in your prescription

It's normal for your prescription to change over your lifetime. Most of us need glasses of some description by the time we're 50, even if only for reading.

If you wear contact lenses, your contact lens and glasses prescriptions may be different. This is because contact lenses are worn directly on your eyes, whereas glasses are worn one to two centimetres in front.

It's also common and normal for your prescription to be different in each eye.

What is 'OD' and 'OS' / 'RE' and 'LE'?

These are abbreviations of Latin terms. 'Oculus dexter' (OD) is your right eye. 'Oculus sinister' (OS) is your left eye.

Some opticians prefer the simpler abbreviations 'RE' for right eye and 'LE' for left eye.

The numbers in your prescription

Your prescription usually contains three numbers:

• 'Sph' or Sphere measurement

• 'Cyl' or Cylinder measurement 
• Axis measurement

What is sphere or 'Sph' measurement?

This describes how short or long-sighted you are. It's measured in dioptres – shortened to D. It can be positive (for example, +4D) or negative (for example, -4D).

What does long-sighted mean?

This means you'll find it harder to see or read something close up. If your sphere number's positive, you're long-sighted. The higher the dioptre number, the more long sighted you are. You'll need to wear glasses (or contact lenses) to correct this, and it's likely to increase as you get older.

What does short-sighted mean?

This means you'll find it harder to see or read something far away. If your sphere number's negative, you're short-sighted. If your number's -4.00D or more, your short-sightedness is quite significant. A number above -0.75D means you're likely to need to wear your glasses for watching television and driving.

What is cylinder or 'Cyl' measurement?

The cylinder measurement describes the amount of lens power for astigmatism. If the front of the eye is round like a football you have no astigmatism. If the front of your eye is shaped more like an oval rugby ball, you will have astigmatism.

Your cylinder measurement is also measured in Dioptres, and can be a negative or positive number. The bigger the number (positive or negative), the more significant your astigmatism is.

What is the axis measurement?

This measures how any astigmatism you have is oriented. It can be between 0 and 180 degrees. This measurement tells your optician exactly where to position your lens.

Bifocal & varifocal correction

From the age of around 45, our eye's ability to focus on things close up deteriorates. This is why many of us need reading glasses as we get older. This is called presbyopia.

For convenience, many people wear bifocal or varifocal lenses, regardless of whether they have worn glasses before. These lenses can provide your existing correction alongside the additional 'reading glasses' correction many of us need with age.

Bifocal lenses have a visible line around the 'reading glasses' part of the lens. Varifocal lenses gradually blend the two segments, with no dividing line.

Next steps

• NHS advice is to have your eyes tested at least every two years, even if your vision seems fine. Many eye conditions don't cause any noticeable symptoms until they've progressed quite far

• Follow your optician's advice about more frequent tests, as they will know the health of your eyes best

• If you have any questions about your prescription, talk to your optician – they'll be happy to explain