PD Measurement and Specification
Because your vision is our primary concern, we recommend that you obtain your PD at the office where your glasses will be made. Our goal is to help you obtain glasses that are correct and safe and made to the American National Standards that professional providers are required by law to meet. Unfortunately, the glasses made by discounters and internet facilities are known to have far too many errors. It’s simply impossible for us to be sure that glasses you get elsewhere will be appropriate. Buy your glasses where a dedicated professional will personally fit them and take all of the measurements necessary to help you get the best vision.
Please review the following to learn about your PD:
What is my PD measurement?
Your PD measurement is the distance between the anatomical centers of your pupils as you view a given object.
What is my PD specification?
Your PD specification is a reference point which must be provided to an optical laboratory so it knows where to correctly position your prescription lens in your frame relative to your effective line of sight.
Are my PD measurement and my PD specification the same thing?
They are not the same thing. Your effective lines of sight can be very different from the measured distance between your pupils. Careless labs tell patients that their PD measurement should be used as the specified PD on their order forms. If everyone’s glasses were made this way, many people would have a hard time seeing and many wouldn’t be able to wear their glasses at all. Specifying the wrong PD can cause double vision, blurred vision, headaches and permanent vision problems.
What is my PD specification used for?
Your PD specification is used by a lab to make your lenses. The lab will change the way it creates and mounts your prescription to conform to the PD specified. Knowledgeable professionals will use your specified PD to help you make lens and frame choices that will improve your vision, balance, comfort and appearance.
How is my PD specified?
Your PD can be specified for distance vision, near vision, looking up or down, or any distance or direction. It can be for one or both eyes. Although an inexperienced trainee will estimate your PD with an automatic machine, a knowledgeable professional with expert technique will specify it more accurately. The purpose of your glasses, your prescription, your lens type, your frame type, the way the frame fits and its distance from your eyes all need to be considered. You must be seen while wearing the frame to get accurate specifications.
Does my PD measurement or specification ever change?
Your PD changes throughout life especially during childhood. It can also change due to disease, injury, prescription changes, postural habits and adaptive behaviors. If someone wears glasses with the wrong PD for too long, they may have trouble wearing glasses with the right PD later. The right specifications don’t matter if the glasses are made wrong, so have all of your new eyeglasses checked at the prescribing doctor’s office.