Obtaining your prescription
Once you have had your eye test your optician will provide you with a copy of your prescription (they are legally obliged to do so). In addition to the normal information provided on your prescription (see below) we also need the PD or Pupillary Distance measurement. Opticians rarely include the PD measurement with the standard prescription details (in this instance they are not obliged to do so). This is the distance between the pupils of the eyes, centre to centre, in millimetres. If you are not able to obtain your PD measurement from your optician it is possible to take the measurement yourself. See the section below.
What a prescription looks like
The first number is called the ‘spherical number’ (usually shortened to ‘Sph’), and this will show whether you are short-sighted (a minus number) or long-sighted (a plus number) in each eye. A higher number (usually greater than -4 or +4) means a stronger prescription and you will need to have thinner ‘High Index’ lenses – these will be a little more expensive.
The second number is the ‘cylindrical number’ (usually shortened to ‘Cyl’). This can be negative or positive and measures the amount of astigmatism you have (astigmatism is when the cornea is more like the shape of a rugby ball than a true round shape).
The third number is the ‘axis number’ which will be between 1 and 180. The axis number describes the curve of each eye.
The PD measurement
As mentioned above, in addition to the normal information provided with your prescription we also need the PD or Pupillary Distance measurement. This is the distance between the pupils of the eyes, centre to centre, in millimetres. Most people have a PD between 54mm and 74mm. If you received two numbers, such as 31/31.5, then these are the distance from the centre of your nose to each pupil centre.
Opticians will not usually include this figure and may not be willing to provide it. Some opticians will provide it but charge a fee (separate to any eye test charge). For adults the PD measurement will not change over time so if you have this figure from the past it is still valid.
Ideally it is best to use an optometrist’s PD value as they have the proper equipment to ensure an accurate measurement is taken. However if you are unable to obtain this figure you can take the measurement with a friend or family member’s help. Have your assistant hold a ruler just below your eyes and measure the distance in mm from pupil centre to pupil centre. Focus straight ahead at a distant object and not on the ruler and keep your eyes steady. Take a few measurements and then work out the average figure.
If you wish you can purchase a PD ruler from us (see our ‘Accessories’ section) to help you take the measurement. If you purchase this ruler from us and subsequently purchase your eyewear from us we will refund the cost of the ruler.