Understanding your prescription

Interpreting your prescription can be completely baffling. Understanding how the numbers on it relate to our day-to-day can be hard too. There’s sometimes understandable anxiety about it, but there needn’t be. Read on to learn more about:

  1. The most common vision issues and their causes
  2. How different lens shapes help by bending light
  3. What the numbers on your prescription actually mean

Common vision problems

Most prescriptions will describe the extra refraction needed to correctly focus light on your retina, depending on your specific needs. (Refraction is just a fancy word for bending light).

Different eye conditions require different lens shapes to bend light in the right way. Your prescription is the key to choosing and using the right lens.

There are three highly common vision problems: Myopia, Hypermetropia and Astigmatism. The diagram below describes how they relate to a normal eye.

Lens performance under common vision conditions

Myopia (short sightedness): light is focussed in front of your retina, possibly due to a strong lens or an elongated eyeball, making it hard to focus on things far away.

Hypermetropia (farsightedness): light is focused behind your retina, possibly due to a weak lens or a shortened eyeball, making it hard to focus on things up close.

Astigmatism: This can accompany both Myopia and Hypermetropia and is also quite common. Light is focused on several points on your retina, making hard edges visually fuzzy.

For extra cred, here’s one more.

Presbyopia: This is typically an age-related problem, occurring in people aged 45 and up, due to stiffening of the eye’s lens. Light is not focussed at any point on the retina, making it hard to focus on things both near and far.

You’re one step closer to interpreting your prescription!

A glass prism refracting a ray of light. Spectacle lenses use the same principle. Image: Han Cheng Yeh, Flickr

Lens shapes

Now that we know some of the possible problems we can look back to our old science text books to solve these refraction errors. You might remember, differently shaped lenses will focus or spread rays of light. So if we need to pull the light closer, we can use a convex () shaped lens. If we need to push the light further, we can use a concave )( shaped lens.

For most people, the amount of pushing or pulling will be different for each eye, as the two eyes age and develop independently, even though they work together to give us vision.

That’s pretty much it, for most people. A simple matter of pushing or pulling light as it passes through the lens in your eye.

The two main shapes for (-) and (+) powered lenses

Your Prescription

Knowing that we need to push or pull the light is a great start, but by how much should we push? We don’t want to judge it by eye, or do we? It turns out, we actually do.

At your last eye exam, your optometrist measured how much correction you need in order to see things clearly, both near and far. Those measurements are what’s written on your prescription. Your prescription acts a bit like a stencil, telling us how much or how little we need to bend the light entering your eye. Once we take your measurements, we can get to work.

Now that we’re all rocket scientists, let’s dig into your prescription and make some sense of it.

SPH

The first value to look for is called SPH, which is measured in Diopters Sphere (DS). This refers to the lens Sphere thickness. Thicker lenses bend light more than thinner ones, generally. This is the basis of your prescription and determines how thick your lenses need to be.

  • For short-sightedness, you will see (-) SPH values e.g. -1.25 or -0.50. These lenses will be thicker at the edge.
  • For long sightedness, you will see (+) SPH values e.g. +0.75 or +1.50. These lenses will be thicker at the centre.

CYL

The next value to look for is the CYL or Cylinder value, also measured in Diopters Sphere (DS). This refers to the cylinder needed to correct for astigmatism problems – quite a common issue, though you may not require this. This is a secondary measurement, though it can change the appearance of the lens thickness in parts.

  • You will typically see (-) CYL values, e.g. -0.50 or -1.25. These lenses will be cylindrical on the inside surface (closest to your eye)

AXIS

If you do have a CYL value, you’ll also see an axis value next to it, different for each eye. This is the degree of rotation needed to align the cylinder in front of your eyes. This can have an impact on lens thickness, though considerably less than the CYL value.

  • You will typically see a number from 0 to 180, e.g. 137 or 51.

ADD

Finally, you may also see an ADD value, again measured in Diopters Sphere (DS). This value is literally ADDed to the SPH value to determine the refraction needed for near vision, important for helping people correct for Presbyopia. Prescriptions with ADD values can be used to create distance glasses (SPH), reading glasses (SPH + ADD) and multifocal glasses (SPH at the top of the lens, SPH + ADD at the bottom of the lens), giving you perhaps more of a headache than they relieve.

  • You will typically see (+) ADD values, e.g. +1.50, +2.00

Typical Prescriptions

Typical Myopia Prescription
21-year-old Juan has a mild Astigmatism in the Left Eye, rotated to 24 degrees. The Right eye is slightly weaker, only requiring a SPH correction.
R -1.25 DS
L -1.00 / -0.50 x 24

Typical Hypermetropia Prescription
50-year-old Susan has an Astigmatism in both eyes, with both eyes also requiring a moderate correction for near vision.
R +1.50 / -0.25 x 109
L +2.00 / -0.50 x 50

Typical Presbyopia Prescription
50-year-old Phillip has Presbyopia, making it difficult to see objects both near and far, clearly. This prescription is suitable for distance, reading, multi-focal and bi-focal glasses.
R +1.25
L +1.50 / -1.00 x 65
Near Add +2.00

Phillip’s Prescription Converted for Reading Spectacles:
R +3.25
L + 3.50 / -1.00 x 65

Note the Add value (+2) is added to the SPH value for both eyes. 

Other Values

Other values you might spot include PRISM and Visual Acuity (VA). You may also notice a PD (Pupillary Distance). This is very important for fitting your lenses and is a simple measurement of the distance between your eyes.

And we’re done. Now you know what SPH, CYL, AXIS and ADD values refer to, that each eye will likely need different lenses and how a lens works (in principle).

Lenses do occur naturally, though they're not much help for reading the paper. Image: Dan1984, Flickr

I can see clearly now

A pair of spectacles can be primitive, whilst making such a profound impact on our lives. Without glasses, you might still be able to write this line, but struggle to see which number bus is fast approaching. Or you may have the opposite problem, with eagle-eyed distance vision yet unable to read the menu in a dimly lit restaurant.

We all see things differently but we all share this one experience — vision impairment can have a hugely negative impact on our lives. Getting your eyes tested and understanding your own prescription are the best first steps you can take.