If you’ve always wondered what all those numbers actually mean when you have your eyes tested, read on!
We’ve put together a short guide to understanding short-sightedness, long-sightedness and astigmatism from the numbers on your glasses prescription.
What does your glasses prescription mean?
In simple, concise terms, your spectacle prescription is a measure of how short-sighted or long-sighted you are in each eye, as well as how much astigmatism you have and which part of your eye is more curved than the other.
Wait, what’s astigmatism again?
This causes a lot of confusion! The cornea, or front surface of your eye is like a clear dome allowing light entering your eye to be focused on your retina. The exact curvature of your cornea determines whether you are short- or long-sighted. Short-sighted folks can see perfectly well up close, but distance is blurred. If you’re long-sighted, close work is more blurred, although distance is often (but not always) fine. A cornea that is more curved than normal is myopic – the medical word for short-sighted. A flatter than average corner is long-sighted.
There. Easy so far.
Astigmatism is simply the difference in curvature between the steepest and flattest curves of your cornea. If you think of the front of your eye as a bit like an egg or rugby ball, one contour is more steeply curved than the other. In the eye it is very subtle, but even a small difference in curvature means you have this condition. It is perfectly normal to have a small amount in each eye. In fact, it is much rarer to have perfectly spherical eyes, with no astigmatism whatsoever.
The difference in curvature often happens naturally, due to the flexible eye’s position in a socket between your rigid brow bone and cheek bone as well as pressure from the eyelids. It gets ever so slightly compressed so it is a little curvier vertically compared to horizontally.
Now you understand astigmatism better than 9 out of 10 medical students (no kidding).
No time to read the whole article? Download the free guide to Myopia Control here.
Eye test results explained
When it comes to the numbers on your spectacle prescription, they are shown for right eye (indicated as RE or sometimes OD – the Latin for oculus dexter, meaning right eye) and left eye (LE or OS – oculus sinister). Most optometrists use RE and LE but you will sometimes come across OD/OS in eye hospitals and some ophthalmologist’s practices.
If there is only one number, this means the eye is spherical and has no astigmatism correction:
RE: + 4.74DS
The unit of measurement is called the dioptre (D) and in this case, DS means dioptre sphere.
So far so good!
The plus (+) sign in front of the number means you are long-sighted. Your glasses lenses will be thicker in the centre and thinner at the edges and make things look bigger (including your eyes).
A minus sign means you are myopic (short-sighted). Your glasses lenses will be thicker at the edges and thinner in the centre and will make things look smaller (including your eyes).
The following prescription is for a short-sighted eye with no astigmatism correction:
LE: - 6.50 DS
What about the other numbers?
Most eyes are not spherical, so the numbers need to be able to show how much more curvature is in one meridian (or contour) and where that is.
RE: -3.00 DS / -1.00 DC x 90
In the example above we can tell that one curve of the cornea is 3 dioptres short-sighted.
RE: -3.00 DS / -1.00 DC x 90
BUT, the other curve of the eye is an extra 1 dioptre short-sighted on top of that 3. This means the opposite contour is 4 dioptres shortsighted (the original - 3 and the extra – 1).
RE: -3.00 DS / -1.00 DC x 90
And the last number? This extra curvature is required at 90 degrees in order to correct the eye fully. DC stands for the extra dioptres (called dioptres cyl).
What about reading vision?
Up until 45 years of age, your prescription is usually the same for all distances. (There are some exceptions to this, including children who are highly long-sighted).
If you’re over 45 years of age, your reading prescription will be different to your distance. This is often shown as a single number (called a ‘reading add’ or ‘near add’) beneath the other information and is always a plus (+) number:
RE: +1.75 DS
LE: +1.75 DS
Reading Add: +2.00 DS
In the above example, the right and left eyes are long-sighted and they also need additional reading power for close work of 2 dioptres.
If you have distance and reading glasses, the distance glasses will be +1.75 dioptres in each eye and the reading glasses will be +3.75 dioptres in each eye. If you have multifocals, the top half of the lens will be +1.75 dioptres and the bottom part (where you look through to read) will be +3.75 dioptres.
The reading add is determined by the distance you read at, so you may have a different prescription for computer work.
Are there any other numbers - what about prism glasses?
Very occasionally there will be another set of numbers. This happens if the two eyes are not working properly together and causing double vision. In this case, the lens can be slightly wedge-shaped to redirect the light and line the two images back up so you can see clearly in stereo without the double vision. This wedge-shape is called a prism. Prism is also measured in prism dioptres (denoted as the Greek letter delta - a little triangle) and prism glasses have another set of numbers on the prescription showing how much prism and whether it is needed to align the vision vertically (base up / down) or horizontally (base in / out):
RE: +1.75 DS 1.0 ∆ Base up
LE: +1.75 DS 1.0 ∆ Base down
So, now you know what the numbers on your prescription really mean. One last thing to keep in mind is that your optometrist may not prescribe exactly what they find. For example, if you have a very small amount of astigmatism and it has never caused you any symptoms, they may not include this correction in what they prescribe. This is standard practice and often makes it easier to adjust to your new glasses.