Eye test results
An eye test uses several measurements to describe your eyesight & the strength of any vision correction you'll need.
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. One of the possible causes could be presbyopia. Presbyopia is just a natural part of ageing where we might struggle to focus at a normal reading distance. Find out more about presbyopia.
If you notice that the prescription for your contact lenses are different to your glasses, 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.
Understanding the health of your eye from your prescription
So you’ve just had your eye exam and you’d like to understand what all the numbers and symbols on your prescription mean. Have a look below to understand their meanings:
The letters in your prescription
OS – These are abbreviations of Latin terms. 'Oculus sinister' (OS) is your left eye. (Boots uses L rather than OS)
OD – These are abbreviations of Latin terms. 'Oculus dexter' (OD) is your right eye. (Boots uses R rather than OD)
OU – involving both eyes
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
Numbers – the higher the number, the stronger the prescription
Plus sign (+) – if you see a plus sign next to a number this means you’re long-sighted (so it’s harder to see things up close)
Minus sign (-) – this means you’re short-sighted (it’s hard to see things far away)
Prism – this is to help correct any misalignment of the eyes
Base – this is whether the prism directs light up, down, left or right
Add – additional magnifying power for near visions such as reading
S (Sphere) - 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. 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.
C (Cylinder) – 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.
Axis – 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 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.
What’s next?
The NHS advise you 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. It’s always best to 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 as they will be more than happy to explain.
