What does my optical prescription mean?

We are all born equal but at the same time all of us are equally unique. And that is especially true when it comes to our eyes. In fact, they are so unique in terms of colour, shape and size that they are used to scan for identification like our fingerprints. No two eyes are the same, which is why we all experience difference vision.

When you visit your optician for an eye test, you may well wonder what your glasses prescription means. It can be a little confusing, so in our latest blog we will be talking about optical prescriptions to help you understand what all the numbers on your prescription actually mean.

But first, we wanted to start by briefly explaining the difference between the visual conditions emmetropia and ametropia.

Emmetropia

Emmetropia refers to the refractive state of our eyes, when the focus of the retina is at infinity and distant objects are sharp and in focus. In other words, things appear clear and precise. In this instance, your prescription would show that no correction is necessary.

Ametropia

Ametropia refers to the refractive state of the eye, when the focus on distance objects with the eye in a relaxed state is not focused and may be blurred. In this case a correction is usually necessary. There are different types of ametropia:

Axial ametropia is due to alterations in the length of the eyeball. It doesn’t allow the image to be focused on the retina. Refractive ametropia attributes the ametropia to the shape of the eye, either the curvature of the cornea surface or the lens.

So let’s now look at the numbers that appear on your optical prescription. The higher the number the more correction you would need.

In front of the number on your optical prescription will be either a positive (+) or negative (-) sign. The positive sign denotes that you are long sighted, meaning you can see in the distance but have blurred vision close up. This is also known as hypermetropia. A minus sign in front of the numbers refers to short sightedness or myopia. This occurs when you can see objects close to you but they appear blurred in the distance.

This first set of figures refer to the spherical correction needed measured in dioptres. Sometimes you will see a second set of numbers under the header cylinder. This refers to an astigmatism found during the examination.

A sphere correction only is when the eye is round like a football, so the correction needed is the same throughout the lens in any meridian.

In cases of astigmatism, this refers to the shape of the eye, as it’s not round and can be more the shape of a rugby ball, so the curvature is different at different points of the cornea or lens. Therefore, light is focused at more than one point in the eye. This will only make part of an object clear, therefore another corrective needs to be given within the prescription, which is the second set of numbers.

The third set of numbers is the angle or meridian which the cylinder correction is prescribed. This is noted in degrees and is a number between 0 and 180.

Prism can sometimes be prescribed within the spectacle lenses; this is normally when the eyes need some additional help to work together. The patient can be experiencing double vison or diplopia or has an issue with the muscles in their eyes not working correctly. The prism helps to focus the light, so it falls correctly onto the retina. Prisms are also measured in dioptres and note which base is needed either in, out, up or down.

As we age our eyes start to lose the ability to focus on close objects. They become less flexible resulting in an age-related condition called presbyopia. Although in this digital age more and more people in their 20’s and 30’s experience presbyopia, it affects everyone over the age of 40. Therefore, we need a bit of a boost to our near vision and this is shown on our prescription as an add and is added to the spherical distance prescription. It’s measured in dioptres and is stepped in increments of 0.25.

We hope you now have a bit more understanding of our glasses prescription. Should you have any concerns about the health of your eyes, please do not hesitate to contact us. If you wish to book an eye test with our friendly optometrist Tim, or would like to know more about our OCT scan appointments, get in touch by calling 01858 433 577. If you prefer, you can visit our shop on Manor Walk in Market Harborough to make an appointment. Alternatively, fill out your details on our eye test appointment booking form and a member of our team will be in touch.