Prescriptions | What does all that information mean?

For many of us, an eye examination is more about the immediate concern of whether your current prescription has changed or, indeed, whether you actually need glasses to help you see better, than anything else!

At the end of your appointment your optometrist will issue you with your prescription: A piece of paper with a series of numbers on it that tell us the sort of lens you need to correct your eyesight. It’s rather grandly known as your ‘refractive error’. The perfect eye projects a sharp, clear image onto the retina, known as ‘emmetropic’. If your eyes don’t do this (and most don’t) it’s most commonly because they have a refractive error and that means the image you see is blurry.

You may have heard the terms ‘short-sighted’, ‘long-sighted’, ‘astigmatism’ or maybe even ‘prisms’, but do you really know what they mean?

What’s a ‘Normal’ Eye?

The power of your lens is measured in units called ‘dioptres’ and most prescriptions are relatively low in power – for comparison, the NHS considers a complex prescription to be anything over 10 dioptres.

A ‘standard’ eyeball has a focusing power of +60 dioptres. This brings light rays coming from a distance to focus on the retina at the back of the eye which is considered to be 22.22mm in length. Around +45 dioptres is provided by the cornea (the front, curved, clear part of the eye) and around +15 by the lens inside the eye. (See diagram 1). Problems occur with this system if any of these numbers differ from the norm.

Short-sightedness

Let’s start with the most common problem: Short-sightedness. Short-sightedness is correctly known as myopia, which means you can only see objects close to you: The closer you have to be, the more short-sighted you are. If you have myopia then objects at a distance create a blurry image on the back of the eye. (See diagram 2) This happens because the parts of the eye that bend the light rays are too strong or the eyeball is too long. When you get closer to the object, the light rays arriving at the eye start to diverge, which counteracts the ‘over convergence’ of the light rays and pushes the focus back onto the retina. Diagram 3 gives two examples of different levels of myopia.

Someone who has a myopic prescription will have a minus figure at the start of their prescription, meaning the lenses in their specs (or contact lenses) are diverging: This counters the eye which is effectively over-plussed. (See diagram 4)

Long-sightedness

Now let’s get onto the more difficult one! Long-sightedness! Hyperopia (sometimes called Hypermetropia) is more difficult to explain. If short-sightedness means you can see things up close, then being long-sighted means you can see things far away, right? Well, not really… although sometimes, yes, that is true! Bear with us!

In this case those light rays coming from a distant object are focused behind the retina (diagram 5). This happens because the parts of the eye that bend the light are not strong enough and the eye effectively doesn’t have enough power to bring the rays to the correct point on the retina.

Some people with a relatively low amount of long-sightedness can see objects clearer if they are really far away – for example, we hear of sailors saying they can see a buoy on the horizon, but they struggle to see the TV, or an HGV driver who can see motorway signs in the distance but not the train times on the monitors at the station.

Someone who is long-sighted will have a plus figure at the start of their prescription, their lenses are plus powered (converging). This compensates for the fact that the eyeball is too short. (See diagram 6)

Astigmatism

Ok, now let’s get into the tricky diagnosis: Astigmatism. If you have an astigmatism then your eyes don’t produce a single point focus, but actually produce two ‘line foci’. Effectively the eye has two different powers at 90 degrees to each other. This is caused most commonly by the cornea having two different curvatures. In an astigmatic eye the curvature of the cornea will vary: It will have two different curves, one steeper than the other, at 90 degrees to each other.

These curves mean that the eye may be more shortsighted in one meridian than the other, or more longsighted in one, or even long sighted in one and shortsighted in the other, which, as you can imagine, requires a more complex prescription.

Prism

Our eyes produce two images and send these back to your brain. These images are also upside down just to make things more complicated! So how do we see one of everything, the right way up?

Well, that relies on the precise co-ordination of both eyes and the fact that they both line up and point at the same object at the same time. The eyes do in fact produce two very slightly different images and that is how we perceive depth properly, but the difference between them is only slight. The images have to fall in corresponding areas on the right and left retinas for you to only see one of the object you are looking at. If the images fall partly or wholly outside these areas, the brain cannot match them up properly and they become blurred, or even double. The direction of gaze and how well controlled it is, is governed by six muscles around each eye which move left, right, up, down and obliquely. If you have a problem with one or more of those muscles, then it can mean that the images produced become too different from one another.

As the brain senses this mismatch it will tell the muscles to work harder to overcome the problem, sometimes they can but often this will lead to eyes feeling tired (it’s hard work!) or achy. Sometimes this can cause headaches too.

If we detect this issue in an eye examination, then in some circumstances we’ll prescribe prism. This allows the eyes to take up their comfortable position, thus removing the symptoms; and shifts the images on the retina, lining them up so the eyes don’t have to work so hard. In the vast majority of cases you wouldn’t know someone with prism has this correction as the deviation of their eyes is so small it’s not cosmetically noticeable.

You don’t need to worry too much about what your prescription says – that’s our job. But you do need to make sure you’re having regular check-ups to keep the information correct. Pop in and see us at any of our branches for an appointment so we can keep your sight crystal clear.