Your glasses prescription explained

Ever wondered what the numbers and symbols on your glasses prescription mean?

Read this guide to understanding your optical prescription.

- or +

– Refers to short sighted prescriptions (Myopia)

+ Refers to long sighted prescriptions (Hypermetropia)

Short and long sighted

If you see a (–) symbol next to or above a number in the (SPH) section - you have a short sighted prescription. Although you can normally see things close up or sometimes really close up. Far away distances would be a blur.

If you see a (+) symbol in the (SPH) section next to or above a number, you have a long sighted prescription. This doesn’t always mean you can see things far away and need a correction for close up tasks such as reading. As sometimes long sighted people struggle to see things far away too.

The phrases simply refer to where light when entering the eye focuses in relation to the retina (the light sensitive part at the back of your eye).

Far sighted the light focuses too far or if you are short sighted the light focuses too short.

Dioptre

A Dioptre is main measurement used in optics and is based on focal lengths measured in meters. For example a 1.00 Dioptre spectacle lens will focus 1 meter away while a 10.00 Dioptre spectacle lens will focus in 0.1 meters (10cm)

If you fancy working out the focal length of your own prescription simply use the equation below:

1m divided by the power of your prescription (found in the SPH part).

Example: If you have a 2.50 in this part, 1 divided by 2.50 = 0.4m (40cm)

SPH (Sphere)

The first box you find on your prescription is the SPH (Sphere). This is known as the lens power. The higher the number the stronger the spectacle lens required to correct your vision. These numbers rise in increments of 0.25 Dioptres with either a + or a – symbol in front of or above it. Sometimes if you require no power you may sometimes find a ∞ symbol (Infinity) this is taken as 0.00 (No power required)

If your prescription has been hand written, sometimes the optometrist misses off the decimal point, don’t worry as its normal and can still be read. Normally the decimal point is placed two numbers in from the right. Examples below:

-175 = -1.75

+050 = +0.50

+225 = +2.25

CYL (Cylinder)

This tells you how much astigmatism you have. Astigmatism is very common and found on most prescriptions and simply refers to the shape of your cornea (the front part of your eye) If your eye is spherical (like a football) you will only find numbers in the SPH (spherical) section and sometimes followed by a DS which means Dioptre sphere. If it’s not completely spherical (like to rugby ball) you’ll find numbers in this box too. The higher the number the more your eye is not spherical.

AXIS

This is the position your astigmatism (CYL) is at and at what axis. This ranges from 1 to 180.

ADD

This part of your prescription is the additional power needed to see things for reading.

Unfortunately as we get older (around mid 40s) small writing becomes more difficult to see and we start to hold things further and further away to read – This is totally normal and is known as ‘Presbyopia’ a normal aging process of the eye, where the natural lens inside the eye becomes less elastic and less accommodative.

These numbers will increase as we get older and need more additional power to read things close up. This will always be a + symbol as it’s an additional power added onto your spherical power when requiring a correction just for close up tasks.

PRISM

Not all prescriptions have this part filled in; this is because prisms are used for muscle imbalances, where the eyes aren’t moving and working together – resulting in either blurred or double vision. If you have this part filled in you will always have the base part filled in as well.

Read more about muscle imbalance here…

BASE

A Prism is a triangle Δ - light always bends towards the Base and will normally be written as either IN, OUT, UP or DOWN but can sometimes be written as a number between 1 to 360.

PD – Pupil Distance

Pupil distance (PD) refers to the measurement between your pupils in mm. This does not form part of your prescription and usually isn’t found on your prescription given to you from your optician. They are not obliged to give this to you and it’s only needed when having new glasses made up.

It’s the responsibility of the dispensing practice to do this for you as part of dispensing glasses.

Read more on how we measure your pupil distance when buying glasses from ourselves.