First things first

Now, a quick lesson in how your vision works

Next, a quick lesson in short sight and long sight

So... how did my glasses/contacts deal with all this stuff?

And what do those numbers on my prescription mean?

Congratulations!

First things first

Go back and read Eye Anatomy for Dummies if you haven’t already. Really, it will help. If you’re too lazy, fine, we’ll try to make it easy anyway.

Now, a quick lesson in how your vision works

The eyes are designed for light to focus right on the retina (the part at the very back of your eye). It has to pass through the cornea first (if you remember from the anatomy section, the cornea is the clear part at the front that provides about 2/3 of the power to focus the light), and then it hits the lens which uses its muscles to manipulate it (that’s the other 1/3), and then it (hopefully) hits the retina all nice and focused. If it does, you see well, if it doesn’t, you don’t. Simple.

Now, for it to work, first, the cornea needs to help, in two basic ways: (a) It needs to be symmetrical. If it’s not symmetrical, that’s a problem, because light’s going to get sent in a couple of different directions. (Lack of symmetry could be a natural problem called astigmatism, which if you have it you’re probably wearing glasses.) Then there’s (b) it needs to be smooth. If it’s not smooth, that’s a problem. Of course, no one has a perfectly smooth cornea — there’s all sorts of microscopic irregularities, but for the vast majority of people those irregularities really don’t affect your vision in any perceptible way at all. But... when you’re aiming a laser at the cornea, it can and does leave the surface bumpier than it found it. The problem with bumps is, they each bend the light different ways. Light rays can no longer focus on the retina, they’re sort of all over the place. — So, long story short, if the cornea ain’t smooth, vision is going to get really weird — you might even see two of everything.

Second, the lens needs to help. Again, it needs to be symmetrical. If it’s not, that’s a problem. You could develop a naturally asymetrical lens — that’s got a fancy name, lenticular astigmatism, and those of you who wear fancy toric contact lenses, this might be why. Also, the muscles that make the lens flex need to be working properly. If the lens is getting stiff with age, you won’t be able to focus on things close up anymore and may need reading glasses. That’s called presbyopia.

Next, a quick lesson in short sight and long sight

First, short sight. This is where the further away something is, the blurrier it gets.

Some people’s eyeballs grow too long. The eye is all set up and meant to focus light on the retina at the back, right? But because the whole structure is just too long for what the cornea and other parts are designed to expect, the retina’s too far away and those light rays undershoot and end up focusing somewhere in front of it. Presto, short sight. The longer the eyeball, the more short-sighted you are, meaning you can’t focus on things far away. This is myopia.

With other people, their eyeball grows too short. So all this light that’s coming in, well, again, the cornea and lens are trying to focus it right on the retina, but because the whole eyeball is too short, the retina’s too close and the light rays overshoot and focus somewhere beyond the retina. Presto, long sight. The shorter the eye, the more long-sighted, meaning you can’t see things close to you. This is hyperopia.

So... how did my glasses/contacts deal with all this stuff?

Your glasses deal with short sight and long sight by basically tricking the light into focusing on the right spot in spite of the shape of your eye. Glasses are designed to bend the light on the way in and make it hit the right place instead of the wrong place it would hit by itself. Simple, huh?

So, for example, you’re short-sighted (myopic): you can read but you can’t enjoy the view without your glasses. Your glasses, on the side towards your eye, are concave — in other words thicker around the edges and thinner in the centre. If you’re very, very short-sighted, they’re really, really thick around the edges (yeah, I know, you hate that and that’s probably part of the reason why you got laser surgery in the first place). These are called minus lenses because the amount of correction is given with a minus sign (-) in front of it.

On the other hand, if you’re long-sighted (hyperopic), you can enjoy the view but you can’t read without your glasses. Your glasses, on the side towards your eye, are convex — in other words thicker in the centre than around the edges. These are called plus lenses because the amount of correction is given with a plus sign (+) in front of it.

Now, there are two more problems you might need glasses for - ones we mentioned back in the beginning, in fact:

First, astigmatism. This was where either the cornea (outside part) is funny-shaped (not symmetrical) or where the lens (inside part) is funny-shaped.

With long sight and short sight, you kind of have one or the other because they’re, well, opposites — your eye can’t be both short and long. But whether or not you’ve got one of those, you might just have astigmatism too — lucky you! Lots of people have a little bit of astigmatism. It’s not that big of a deal. If you have lots of astigmatism, that’s a little more difficult and you might have to wear funny lenses called torics. If you’re extreeemely short-sighted AND have lots of astigmatism, you probably worship at the feet of any optician who can give you contact lenses that really work well for you.

Second, presbyopia. This is where your lens is getting old and stiff and can’t focus properly on things close to you so you have to wear reading glasses. If you never had to wear glasses to deal with the other nasty stuff, fine, you just pop on a pair of readers when you’re, well, reading. But if you already wore glasses, and now find you can’t read even with them, you need one kind of lens for the usual stuff and another kind of lens to see up close and make up for your decrepit old natural lenses. You can either switch back and forth, or you can get bifocals, glasses that have, you guessed it, two different lenses, one on the top and one on the bottom. Or you could get really fancy with trifocals.

Whew! See, it’s really pretty simple. It’s all about deformed eyeballs and the funky things you have to do to work around them.

Now, contact lens wearers, you’re undoubtedly wondering, "What about me?" Two answers: One, the same general principles apply, i.e. the lenses are designed to bend the light to hit the retina correctly. Two, go ask your optician, because I haven’t got a clue about the details. What do you think this is, anyway, the bible?

And what do those numbers on my prescription mean?

Your prescription should look something like this. We don’t mean the numbers, silly, just the format, so if your numbers are a quarter of this or four times this, don't panic yet.

OD —4.25 —1.50 x 160

OS —3.50 —1.00 x 180

Now let’s decipher the gibberish into something intelligible to normal human beings:

OD versus OS

OD means your right eye and OS means your left eye.

That’s really all you need to know. For the benefit of enquiring minds and to prove to Latin buffs that we know our stuff, it stands for oculus dexter and oculus sinistre. Heh, heh, heh.

Of course, after reviewing your post-operative topographical scans, you will probably simply remember them as "Oh d*mn" and "Oh sh*t".

Or, you could remember that starboard is the right side of the ship and that your eyes work the opposite way.

Or you could bookmark this page in your browser to keep for future reference.

One way or another, you need to know, because although your topography scans will definitely be marked "OD" and "OS", they may or may not also say "right eye" and "left eye".

The first number

The first number is the amount of myopia (short sight) or the amount of hyperopia (long sight) that you have, measured in handy dandy little units called dioptres. If you’re —0.50, you’re doing great but you might notice it’s not perfect. If you’re —2.50, you’re definitely noticing, but you’re not stumbling through the streets if you break your glasses. If you’re —5.00, you’re getting awfully close to the stumbling stage. If you’re —10.00 or more, well, when you wake up in the morning you might not immediately figure out who’s lying next to you.

How do I know whether it’s myopia or hyperopia? If it has a little minus sign (that’s "-", for those of you who have forgotten your arithmetic) it’s myopia, and if it has a little plus sign (+) it’s hyperopia.

The second number

The second number is the amount of astigmatism - how strangely shaped your cornea is. This is always marked with a minus (-).

If it’s more than, say, -1.50 or —2.00, that’s kind of a lot of astigmatism to have and you probably have to wear toric lenses.

The third number

The third number is just a reference number — the axis of astigmatism, to be exact — basically, which part of your cornea is shaped wrong.

Now, which prescription are we talking about, glasses or contacts?

Remember now, your glasses sit on your nose and there’s space between them and your eyes, while contact lenses sit right on your eyes (hmm... could that be why they are called "contacts"?). That means they work a little differently. So, the prescriptions are different. Your contact lens prescription numbers (the first ones, anyway) are going to be lower than your glasses prescription. Now, the vision tests that your eye doctor does are always going to give a result that’s the equivalent of your glasses prescription, not your contact lens prescription, because he’s using glasses lenses not contacts. (If you are a contact lens wearer, he’ll calculate your prescription from the glasses prescription.) So, if you’re looking at the report from your laser surgery and you see the numbers and they look higher than your prescription has always been — please don’t panic. It probably just means that you’re used to looking at your contact lens prescription.

What if your prescription didn’t look like my example?

Suppose it was... OD PLO -0.75x005, OS PLO -0.50x170. The "PLO" has nothing to do with the... never mind. It just means plano, which is the same as 0.00, as in zero dioptres of myopia or hyperopia.

Or suppose it talks about "sphere" and/or "cylinder". Sphere relates to the first number in the prescription -—it describes the way the lens will curve in order to (hopefully) correct your short or long sight. Cylinder relates to the second number and describes the element in the lens that’s going to correct (hopefully) your astigmatism.

Congratulations!

You are now a vision expert. Seriously, you know far more than the average person does about your eyes. — Not to mention far more than you ever wanted to know, and far less than you’re going to know in a few short months.

Copyright 2003 by LaserMyEye Limited. All rights reserved.
No portion of this article may be duplicated in any format without permission from the Author.
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