Understanding your eyewear prescription

Posted on Categories glasses guides

You might feel one of the below when you read your glasses prescription…

1. Reminds me of physics at school: it might as well be in Japanese.

2. It’s not my problem, it’s the optometrist’s. As long as I get my glasses.

3. If someone can explain these numbers to me in under 3 minutes, I’m game for learning the basics.

And the clock starts…NOW!

After an eye examination (which it’s advised you go for at least every 2 years) you’ll be given a copy of your prescription by the optician, a bit like the one below.

The prescription contains all the details needed to order lenses that will make clearer/improve your vision whether for reading, or for general use.

Aside from the measurements that might be provided to you for suitable lenses, the optician will also check your general eye health. But that’s another blog post.

You’ll need your prescription information when click ‘Add to Cart’ on your frames in our gallery. If you have any questions, please just contact us.

Expiry Date

Let’s start with the easy bit.

Australian prescriptions have an expiration date. You won’t be able to order glasses if your prescription has expired so it pays to check before you begin looking for glasses.

Your prescription is generally presented in a table (above). You will have separate measurements for the right eye and the left.

Pupil Distance

There is one thing often missing from prescriptions that is worth asking for before you leave the opticians: your pupil distance.

Your pupil distance is the distance between your pupils. Some opticians assume that, after your eye examination, you will be buying directly from them in their shop before you leave. Hence why they leave it off the prescription. However, to order online/from Sneaking Duck, you’ll need to obtain this measurement to order glasses so we can position the lenses correctly for you.

While you can measure it yourself (check out the video below) it’s a lot easier to have a professional do it for you, especially if you’re already at the opticians. Some opticians can be a bit reluctant to give it to you but we’ve found the ‘sweet but insistent’ approach usually works.

SPH (sphere)

This number represents how long or short-sighted you are. Remember, if you’re long sighted, you have difficulty seeing things close up. If you’re short-sighted, it’s the opposite: you might have trouble seeing things further away.

+ indicates long-sighted

– indicates shortsighted

The larger the number, the stronger the lens you will require and this can mean thicker lenses. The numbers typically range from 0.25 (very small) or 6 (very strong).

We can advise you on different lens options depending on your prescription. A higher prescription can mean thicker/curvier lenses.

CYL (cylinder)

The CYL number measures the amount of astigmatism caused by an irregularly-shaped cornea.

Your cornea sits at the front of your eye and covers you pupil and iris.

No number in the CYL box…your cornea is probably perfectly spherical, like a football which means that you have no astigmatism.

Low number (like 0.25)…you have a cornea that is not quite perfectly-round.

High number (like 3.00)…you have a distinct oval-shaped cornea (if we’re keeping to the sports theme here, think rugby ball) and you may experience visual distortion, which is called an astigmatism.


This is related to the direction of the astigmatism, or the cylinder. It’s measured in degrees from 0-180 and will help the lab know at what angle to position your lenses within the frame.


A small number of people have eye alignment problems, meaning that their eyes don’t work together well as a pair (sibling rivalry?).

If the ‘prism’ box has been filled in on your prescription, prism lenses should correct the muscle imbalance between your eyes by banding the path of light without altering the focus.


This tells the lab where to put the prism (if any) in your glasses.


If you need multifocal lenses (a different prescription at the top than at the bottom to enable you to use one pair of glasses for reading things both close up or in the distance), this box might be filled in.


Hey, you just learnt Japanese/became a physics guru! Well, sort of.


Thanks to Pete, Yandle and Jacobo Perez (Buddha) for the Flickr images!