Understanding your eyesight prescription
Mar 20, 2019 in Eye Tests
We’ve all received a glasses prescription, but what exactly do all the numbers mean? Here’s our quick guide to making sense of your prescription.
Whenever you have an eyesight test at Leightons, you’ll be assessed for your glasses prescription. It’s a precise description of the lens correction your eyes need to see clearly. In the UK, spectacle prescriptions follow a specific format, with several measurements per eye.
What the table means
As your eye test proceeds, the optometrist will take an accurate note of the values measured for each eye.
This is the number you might recognise. If you wear glasses, you probably know your lens power number, either minus eye power or plus eye power. This ‘sphere’ value, measured in dioptres, describes the strength of your glasses and the level of correction needed. If you have a minus number, like -2.75, it means you’re short-sighted and struggle to focus on distant objects. A plus number indicates long-sightedness, so objects up close appear more blurred or close vision is more tiring on the eyes.
The sphere value moves up and down in 0.25 increments, anything smaller is imperceptible. Patients will commonly fall within the -25.00 or +25.00 range.
For higher prescriptions, those over +/- 4.00, we might need a ‘back vertex distance’ (BVD). It’s a millimetre measurement from the back of the lens to the cornea of the eye.
Cyl (cylinder eyesight)
The cylinder value relates to astigmatism and measures the imperfections in the curvature of your eyeball. Astigmatism results in distortion so that the image on the retina is unevenly distributed around the retina, causing blurring and distortion. To correct it, we can use a lens of equal, opposite distortion. The ‘cyl’ value describes these precise alterations.
Axis and cyl work together as a pair – you can’t have one without the other. The axis value tells the lens maker where to place the astigmatism correction (the cyl value). Axis is always a value between 1 and 180 degrees.
Prism and Base
If your eye muscles aren’t working together as a team, we can use the prism and base values to correct them.
The prism value is a number, moving up in 0.5 increments. Base is the direction: up, down, in, or out. It can be applied to one eye or to both.
The difference between ‘distance’ and ‘near’ So far we’ve only looked at the top row, ‘distance’. This is the lens prescription for distance vision, but for reading glasses and varifocals, we also need values in the ‘near’ row.
For reading glasses, we’re adding a magnifying lens to counter the loss of accommodation due to presbyopia (age-related near-sightedness). This is marked as a number, such as ‘ADD +2.00’, meaning ‘add 2.00 dioptres of magnification to the sph value’.
An ‘intermediate add’ can also be included to help with middle distance such as computer vision or reading music.
My vision is getting worse. What do I do? If your prescription has worsened over time, don’t worry, you’re not going blind – it’s just a way to describe the type and strength of lens you need to correct your vision.
The rate of change in your prescription depends on your age. Short-sightedness often develops in childhood, while presbyopia affects everyone around the age of 45, while general changes in eyesight will often plateau and stabilise around 60.
How do I get ‘20/20 vision’?
The short answer – if you wear glasses or contact lenses – is that you probably already have 20/20 vision (if your prescription is up to date). Your prescription is designed to get you as close to normal vision as possible.
And don’t forget that in the UK, we use ‘6/6’, not ‘20/20’ (6 metres = 20 feet). But what does it really mean? It’s deceptively simple: 6/6 means you can see from a distance of 6 metres what the average person should be able to see from 6 metres.
The standard for driving is 6/12 – so you don't need perfect sight to drive, but it’s important your optometrist checks you meet this minimum standard.
And finally, what does this mean for my contact lenses?
Everything we’ve described here is exclusively for glasses. Contact lenses need a separate prescription and a separate consultation.
Only qualified optometrists, OMPs or contact lens opticians can convert a spectacles prescription into a contact lens prescription. Diameter, curvature and other factors need to be carefully considered, along with advice and support on safe lens use.