Deciphering your Optician's Prescription
Created | Updated Jan 9, 2003
Once you've had a sight test you will be given a piece of paper/card with your prescription on it*. To most people this is just a load of numbers which don't make any sense, this article will hopefully explain the main parts of your prescription, so it does. As a point, an Optician is really called an Optometrist.
I'll give examples of prescriptions. It must be made clear that these are only examples and this is not a complete guide to every single possible prescription. Your prescription may have extra things on or not fit in the 'usual' ranges. * If in any doubt ask your optometrist. *
The Science Bit
One of the main functions of your eye is to focus light onto the retina for it to be detected. There are several structures which help in the refracting (bending) of the light so that it focuses at the right distance (ie on your retina). The two main ones are the cornea (the clear curved bit at the front of the eye) and the lens (an elastic structure suspended in the centre of the globe).
The cornea does most of the bending (about 2/3rds) and its power * is pretty much set *. The lens' power can change according to what distance you are viewing. In younger patients the lens is very elastic and can change power to view all but the closest distances. In older patients (45ish and above) the lens starts to become less elastic and so has less capacity to change power to focus at different distances.
In a normal (emmetropic) eye the light is focussed on the retina, in this case no glasses are needed.
In a short-sighted (myopic) eye the light is focussed in front of the retina. This can be because the eye is too big or that the eye bends the light too much. To correct this a negative (or minus) lens is used.
In a long-sighted (hyperopic) eye the light is focussed behind the retina. This can be because the eye is too small or that the eye doesn't bent the light enough. To correct this a positive (or plus) lens is used.
Astigmatism is where the eyeball is no longer a sphere (football shaped) but is oval (rugby ball shaped). This means that two different powers are needed to focus the light onto the same point of the retina. One for the 'length' of the ball and one for the 'width'. The difference between the two powers is called the cylinder (this can be either plus or minus).
The rugby ball can also be rotated, giving a different lens for each degree of rotation. This is the axis.
Example
| Sphere | Cylinder | Axis | Prism | Add | V/A | N | |
|---|---|---|---|---|---|---|---|
| R | +4.00 | -1.00 | 90 | 1∇ OUT | +2.00 | 6/6 | 5 |
| L | -4.00 | +1.00 | 180 | 1∆ UP | +2.00 | 6/18 | 12 |
This can be split into parts
Sphere
This is the amount of plus or minus power needed to focus the light onto the retina. If your eye is football shaped the amount of power needed is the same in any direction. In this case you will only have a number in this box. But if your eye is rugby ball shaped you need the next two boxes filled in to say how different the length and width are and at what angle the change is.
Cylinder
This is the difference between the length and width of your rugby ball*.
The cylinder can be written in two ways, as a positive number or a negative number. This can be quite confusing as the two look quite different when written down. It comes from which value (length or width) you use to measure the difference from.
If you decided to make your sphere value your length (+4.00) to get to your width you would need to add the cylinder value to find the power (-1.00) which would give +3.00. But if you wanted to make your width the sphere value (+3.00) you would need a cylinder value of +1.00 to give your length.
This gives either +4.00/-1.00 or +3.00/+1.00.
They are both the same lens, but written in two different ways*.
Axis
The axis is the angle at which the length is positioned. This is a value between 1 and 180 degrees.
Whilst +4.00/-1.00 and +3.00/+1.00 are the same lens, the axis at which they are positioned is different.
One is the angle for the length and one is for the width. These are always 90 degrees apart. So if one is 90 degrees the other will be 180, 73 and 163, 17 and 107, etc.
So really +4.00/-1.00 x 90 is the same as +3.00/+1.00 x 180.
Prisms
A patient needs prisms to help with double vision. If one or more of the eye muscles involved in moving the eyes isn't working as well as it could, the eyes might look in different directions. This gives the brain two different images, which it can't merge to form one picture. You could try this by crossing your eyes, there are two images, but your brain can't match them up, they're too different. Prisms work by moving the images, in glasses they are used to make them overlap, so the brain can match them up.
Sometimes the other muscles work harder to compensate for the weaker one. This can cause eye strain and headaches
Add (Near and Intermediate)
This is the extra power your eye needs to see things close up as the lens becomes less elastic. When an Add is prescribed and the patient generally has a separate pair of reading glasses or a single pair of bifocals or varifocals.
This value should be between +1.00 and +4.00, but might be a bit more or less according to your prescription. This is usually the same for both eyes, but can be different.
Near is the term used for close work such as reading or sewing and is generally classed as such to about 40cm.
Intermediate is the term used for distances generally longer than 40cm but less than a few metres such as computer screens, musical scores or work desks.
V/A and N
These are both values of similar things. Basically how far down a test chart you can read.
