Understanding your eyesight prescription

Aug 19, 2020 in Eye Tests

Woman having Leightons eye test with phoropter head technology

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.

eyesight prescription table

Right and left are reversed – the table matches the Optometrist’s face-on view of your eyes.

This prescription shows that the patient is hyperopic and needed a plus eye lens to correct their vision.

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What the table means

Throughout your eye examination, your Optometrist will take an accurate note of the values measured for each eye.

Sph (Sphere)

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 find it more difficult 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 and the majority between +4.00 or -4.00.

For those considered to have high prescriptions, those over +/- 4.00, we might also need a ‘back vertex distance’ (BVD), which is a millimetre measurement from the back of the lens to the front surface of the eye.

Cyl (cylinder)

The cylinder value relates to astigmatism and measures the deviations in the curvature of your eyeball. To correct it, we use a Cyl, which is equal but opposite to the eye’s astigmatism. The ‘Cyl’ value describes these precise alterations to help you see.


Axis and Cyl work together as a pair – you can’t have one without the other. The axis value, always between 1 and 180 degrees, tells the lens maker where to place the astigmatism correction (the Cyl value).

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 and can be applied to one eye or to both.

What is the difference between a ‘distance’ and ‘near’ prescription?

So far we’ve only looked at the top row, ‘distance’. You can be long or short-sighted for distance prescription.

If you’re short-sighted, you require a minus power lens and you’ll notice that the lenses in your glasses might look ‘thinner’ at the middle and ‘thicker’ at the edges. If you are long sighted, you require a plus power lens and you’ll notice that the lenses in your glasses might look ‘thicker’ in the middle and ‘thinner’ on the edges.

This is how a lens works for short sightedness:

Your Lens Prescription  for Short Sightedness: A diagram

For long sightedness, this is how a lens works:

Your lens prescription for Long Sightedness: A Diagram

But for reading glasses and varifocals, we also need values in the ‘near’ row.

Varifocals are incredibly clever lenses that allow you to focus on near objects and into the distance.

Varifocal lens

For reading glasses, we’re adding a magnifying lens or ‘plus eye power’ as a solution for presbyopia (age-related loss of near-focus). 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 to assist with hobbies (such as browsing the web on your computer).

Your expert Optometrist will advise which prescription you need. Your Qualified Dispensing Optician will help you select the most appropriate frame for your prescription and talk you through additional extras such as glare support and tints.

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My vision is changing. What do I do?

If your prescription has changed over time, don’t worry. Most likely, we just need to alter the strength of your lenses.

The rate of change in your prescription depends on your age. Short-sightedness often develops in childhood, while presbyopia (age-related loss of near-focus) affects everyone around the age of 45, while general changes in eyesight will often plateau and stabilise around 60.

Remember, your eye examination also checks the health of your eye. We pride ourselves on offering the latest technology including OCT and Optomap. Your Optometrist will be looking at all signs of why your vision may have changed and can offer the best advice for you.

If you are concerned about changes in your prescription, we are open and here to help. Our Optometrists are always on hand to help answer any questions and offer a complete eye examination and prescription as normal at your nearest Leightons practice.

At this time, there may be some differences you may notice when you visit us in practice, including social distancing measures and PPE. All practices have COVID-19 safety measures in place to ensure the very highest standards of service and care, for you.

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 or ‘perfect’ 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 driving standard with regards to letters on the chart is 6/12+ – so you don't need ‘perfect’ sight to drive, but as per government guidelines, 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 specification and a separate consultation. Contact lenses are great if you are required to wear a mask and/or visor and for participating in hobbies without worrying about your glasses getting in the way, fogging up or accidentally breaking. Optometrists or Contact Lens Opticians can convert a spectacle prescription into a contact lens specification as part of a contact lens fitting. The diameter and curvature of a contact lens are only some of the factors to be considered, along with advice and support on safe lens use.

If you’re keen to know more about contact lenses, ask a member of the team.

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Get your prescription updated with your nearest Leightons practice for the highest level of care. The NHS recommends you get your eye health and prescription checked every two years.

Book an appointment by calling our Dedicated Patient Support team on 0800 40 20 20 or book online.

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