What is a PD Measurement and Why is it Important?

 

What is PD?

Pupillary Distance, or PD, is the distance between the pupils of your eyes. When it comes to spectacles, PD is specifically the distance between the pupils when focused at distance and the eyes are in the primary position –so, imagine when you are behind the wheel of your car, staring down the motorway into the far distance.The near PD is when the eyes are focused on something closer, for example, when you’re reading. In this situation the eyes will typically converge so the PD will be smaller.

Why does my optician need to know my PD?

Because of how lenses in spectacles work, your PD measurement is a critical piece of information needed to produce your glasses correctly: Every lens, be it in a camera, microscope, magnifying glass or spectacles, has an optical centre (OC). The OC is actually the only point on the lens where its strength is absolutely true. At every other point the power differs from what was originally ordered. This is due to the physics of how light travels through lenses and is therefore to be expected. However, the further away from the OC (and this is especially so for stronger prescriptions) the more the power differs and the more error you will note towards the periphery of your lens.

So, when we put together an order for your glasses we carefully measure your PD so that we can make sure the OC coincides exactly with your pupil centres, to give you the most accurate and comfortable spectacles.Incorrectly centred lenses can lead to blurry, uncomfortable vision or even double vision. The alignment of the centres is always important but, again, gets increasingly so the stronger the lens is or if you need a multifocal.

Once the lens is glazed to the frame the OC becomes a fixed point, but obviously our eyes are constantly moving so while it’s not the case that we are always looking through the OC, it is crucially important that the alignment is correct for things like driving or watching TV. If you are a glasses wearer, you may recognise that strange feeling when you first got glasses or had your prescription changed –when things just suddenly looked and felt a bit odd? This is due to your brain becoming accustomed to and tuning out the error from the periphery of the lenses as your eyes move around. (How our brains process our vision is a subject all of its own – we’ll expand on that in a later blog!)

Can you give me my PD so I can order glasses online?

We often get asked why we don’t note your PD on the copy of the prescription that we provide after an eye examination. This is partly because it isn’t a legally required part of a prescription but also because it is information which should only be measured by the supplier making your spectacles: Obviously it’s impossible for an online glasses supplier to measure your PD and doing it yourself is tricky and unreliable.

So, at Patrick & Menzies our policy is not to provide a PD measurement on request: Your PD will vary depending on the fit of the frames you choose and if we provide a PD measurement which is then used to buy glasses online, some of the responsibility of whether those glasses are correct or not, falls to us. However, if we have had no opportunity to assess the fit and suitability of the frame, or your lenses, let alone be able to check them upon manufacture, then we’re sure you will understand that we wouldn’t feel comfortable taking that responsibility on.

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