The debate about the p.d continues to rage on in Optics! When you have an eye test and they either use a ‘trial-frame’ (that odd looking pair of adjustable specs that opticians use) or the automated machine that swings down in front of you with all the lenses inside it (phoropter), the Optician must set the instrument up with the correct p.d to be able to test you properly, so it is routinely measured during a sight test, but it is never noted! So much so that if you request the measurement in most cases it is refused.
The p.d is needed so that the optical centre (the optimum vision point of the lens) is positioned directly in front of your pupils when your wear the specs as intended. This is very important both for visual comfort and clarity. If this is set incorrectly there may be visual consequences the higher the prescription the more important it is.
Some online retailers use average values depending on the gender of the customer, this may mean that the p.d could be many mm out! Some will give you videos and demonstration how to measure it yourself. It isn’t difficult to do, but it does need to be accurate. You can see this on You Tube at:
There is even now an ‘app’ for the iphone for measureing the p.d it will cost £3 to download and seems to work quite well. http://itunes.apple.com/us/app/id381060059?mt=8
(Source: freshoptical.co.uk)
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