What Do the Numbers in Your Prescription Mean?

Refractive errors are very common, with more than half of us being affected. We saw above that the effect of a refractive error is an image that falls somewhere other than exactly on the retina. A lens in front of the eye is needed in order to bend the light rays inwards or outwards so that the image will, in effect, be pushed back or pulled forwards until it lands in the right place. Your retina at the back of your eye can then detect the image details and send them off to your brain.

The larger your refractive error, the further the image is away from the retina and the stronger the lens needed. The strength of this correcting lens is measured in units called dioptres (D). So someone with fairly mild shortsightedness might need a lens of one unit of strength (minus one dioptre, written as -1.00D), where someone with much worse myopia, five times as bad, would need a lens of -5 dioptres (-5.00D). As these prescriptions are without any astigmatism, a round or "spherical" lens is used to correct them (written as -1 or -5 “dioptres sphere”, or DS for short = -1.00DS and -5.00DS respectively).

Let's look at a prescription. Long-sighted, with a small amount of astigmatism in the left eye:

(When we write this quickly, it usually looks like this: +4.00/+0.50 x 90)

The three numbers on the prescription then are:

1. The basic correction, written as Sphere or SPH (because we correct it with a “spherical” lens). The higher this first number, the worse the vision and the thicker the glasses will be. A minus means shortsight and plus means longsight

2. The amount of extra distortion from astigmatism, on top of the basic blurring. This is written as Cylinder or CYL because we use a so-called “cylindrical” lens to fix it.

3. The precise angle this cylinder lens should be tilted to give the sharpest vision, written as Axis (between 1 degree and 180 degrees – 180 is horizontal and 90 vertical).

Presbyopia

If you are over the age of 40 you might also need reading glasses (presbyopia), or wear bifocals (glasses with a distance lens above and a smaller lens at the bottom with some extra power for near work). The additional power needed for reading is called the "Add", and is usually written as a fourth figure after the main distance prescription (not surprisingly in the column labelled Add!) In your mid 40's this might be an extra +1.50 dioptres, but this increases as you get older to perhaps +2.50 or even +3.00 for the over 60's.

Your Best and Worst Vision

When you are considering laser correction, you must think about your eyesight in two ways. The first is your poorest vision, the one you have when you aren’t wearing your glasses or contact lenses. This is your uncorrected visual acuity or UCVA for short. The other is the best vision your eyes are capable of when you wear spectacles, what we call the best (spectacle) corrected visual acuity or BCVA for short (and sometimes BSCVA).

Obviously there is a wide range of UCVA which will depend on the amount of your refractive error. If you are myopic with a prescription of -1.00DS, your UCVA might be just good enough to drive without lenses. But if you are -5.00DS you might be able to find the car but not much else! I had a patient who confessed to driving without his glasses even though he was -3DS and could barely see 20/200! I asked him to telephone me every time he was going out, so I could make sure I wasn’t on the road at the same time.
There is a much smaller variation in the level of BCVA, with everyone clustered at or somewhere not to far from 20/20, assuming the eyes are otherwise healthy. Simply put, when people have their glasses on or their contact lenses in, there isn’t too much difference in their level of vision.

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