The A-Z of an eye test

What are optometrists measuring during an eye exam?
10 April 2018

Interview with 

Dr Keziah Latham, Anglia Ruskin University


eye test


Regular eye tests are a good idea for keeping tabs on your vision, but how do glasses actually aid your sight, and what are optometrists looking for when they shine those torches in your eyes? Katie Haylor took a trip to Anglia Ruskin University’s eye clinic to meet optometrist and reader in hearing and vision science, Kez Latham.

Kez - An eye test is very much a routine part of your health maintenance. A regular eye check, in the same way as a regular dental checkup, is a really important thing to make sure that your eyes are working as well as they can be, and to spot any signs of things going wrong at an early stage.

There’s two main parts to an eye examination: the first part is to check the refraction of the eyes; we want to make sure that you can see as clearly as is possible and find out what lenses, if any, we’d need to put in front of the eyes to help you see as well as possible. The second part is to look at the health of the eyes.

Katie - Well, I think I’ve put it off for long enough now; I’m going to have to get in the chair so shall we crack on… It’s quite comfy actually.

Kez - What we want to do to start with is to see how well you’re seeing with your current specs as you're a spec wearer.

Katie - F, H, V, N, H. D, E, F…

By reading the lines of letters which got smaller and smaller further down the chart, Kez was establishing my baseline eyesight. And from there she could do more detailed tests in order to establish how well I can see. But what are my eyes doing with the light that comes into them?

Kez - When light comes into your eye from an object that’s in the distance, the surfaces of the eye converge that light so that the light comes to a focus. First of all and primarily, the cornea, the front surface of the eye, has the most impact on bending the light.

And then, secondarily, the lens that’s within the eye has another effect on converging the light so that it comes to a focus. The lens has slightly less of an effect overall in terms of the power of the eye, but the crucial thing about the lens, in your eye at least, is that it can change its power and that’s very helpful when you look from looking in the distance to looking at close up, the lens in your eye changes power so that it can cope with the fact that the light is more divergent from a nearer object.

Behind the lens we have the posterior chamber of the eye filled with transparent material, because we want the light to get all the way through to the back of the eye until it reaches the final destination which is the retina. And that’s where all the magic happens because that’s where the light energy that’s reaching the back of the eye is going to get converted into nervous energy that then gets passed up to the brain so that we can see.

Katie - People often say they’re eight long-sighted or short-sighted, what do these terms actually mean?

Kez - Somebody who’s long-sighted has an eye which isn’t quite powerful enough, so the rays of light converge to a focus but not quite quickly enough so that focus actually forms behind the retina. So we need to move that focus a little bit further forward by adding a slightly more converging lens in front of the eye to bring that focus forward onto the retina.

Now, on the other hand, short-sightedness is where the eye is slightly too powerful for it’s length, so light coming into the eye is refracted or converged and the eye’s a bit too efficient at doing that. The rays of light are converged a little bit too much and they come to focus in front of the retina and then the light rays carry on, and by the time they reach the retina the image is blurred again.

Katie - So short-sighted people need diverging lenses in order to push the point of clearest focus backwards onto the retina. Whether you’ve got glasses, contacts, or even had laser eye surgery, the aim of the game is the same; it’s to adjust the way that light bends in the eye in order for it to fall spot on onto the retina.

There are other tests involved in an eye exam. Through assessing my vision with a series of lenses, Kez checked if I had astigmatic or rugby ball shaped eyes that can cause visual impairment. Other tests included checking if my eyes were working together well as a pair, and if the muscles around the eyes were doing their job well enough to allow me to move my eyes in all directions.

But it’s not just how well we can see that’s covered in an eye test. Optometrists also look into the eye itself.

Kez - There are certain specific structures that we want to pay particular attention to. The first thing that we look for when we look into the back of the eye is called the optic nerve head, where all the nerve fibres from across the retina gather and go up the plug hole to the brain up the optic nerve.

Katie - Can I put my glasses back on now? Oh gosh, they’re embarrassingly dirty.

By closely examining structures in the eye like the optic nerve head, optometrists can potentially pick up on changes indicative of certain conditions; for instance eye conditions like glaucoma, or systemic conditions like MS. But it’s not just the optic nerve head that’s important to check out, so once I got my glasses back on and the world became clear again, Kez told me about some other things that she looks for…

Kez - We also want to look at the macular in great detail; that's the area at the back of the eye where light is focusing, and so it’s the area of our eye with finest vision. When we look straight at something, it’s our macular or our fovea that we use for all sorts of fine resolution tasks like reading, recognising faces, reading car number plates, those kinds of things.

The other thing that we particularly want to look at, as well as the rest of the retina and any other changes that we’re looking for there, we also want to have a careful look at the blood vessels at the back of the eye. At the back of the eye is the only place in the body that you can see blood vessels directly without opening the skin, so it is very important to look at those blood vessels as being indicative of somebody’s general health. You can pick up signs of systemic diseases: things like high blood pressure, high cholesterol, and even diabetes by looking at the condition of the blood vessels at the back of the eye.


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