Your PD is the distance between 2 imaginary lines running through the centre of your pupils. When you are looking at something far away these lines are parallel and your eyes as are as far apart as they can be.

When you are reading then the two imaginary lines cross at the distance you are reading at – i.e. where you are holding your book. When you are doing close work your eyes are converging together so the near PD is smaller than the distance PD.

In practice if we have your distance PD we can calculate your near PD based on a working distance of 40cm.

Your PD measurement is used to position the "optical centres" of your lenses correctly for your individual requirements. On many modern lenses it is also necessary for us to measure the optical centres vertically as well as horizontally – this can only be done with the frame in place.

The higher your prescription the more important it is that we have an accurate PD measurement. On high prescriptions an error in the optical centres or PD, can cause you to see double, or more likely cause eye strain and headaches. This is why ready-made reading glasses are only available in low powers, and even then often cause headaches.

However if you feel your eyes are wide set or close together compared to the average you should endeavour to provide us with an accurate measurement for optimum comfort, or state "smaller than average PD" etc on your order so that we can make an educated allowance to optimise your glasses to your needs.

Ordering your glasses online or wearing ready readers is likely to produce an inaccurate PD. Having an incorrect PD measurement is unlikely to harm your eyes, however if your optical centres are positioned even slightly differently to your pupillary distance you may experience discomfort or eye strain. In extreme cases you could experience occasional or even permanent double vision.

You wear your glasses every day and your eye sight is precious so why put up with even a small amount of discomfort? Coming to Conlons is the best way to ensure your glasses are as comfortable as possible, however even if everything has been measured perfectly you may still need to adapt to your new prescription for a week or two.