Knowing your PD is important when ordering prescription eyewear. PD stands for "Pupillary Distance" and is the distance between your pupils. To understand why your PD is important, it's useful to know how prescription lenses work.
How Prescriptions Lenses Work
You might remember from your high school physics classes that glass can affect the direction and focus of light. Depending on if your glass lens is convex or concave, the path of light can be altered and focused. The degree of which these lenses were effective for redirecting and focusing the light also relied on at what point of the lens light was shone through.
In a similar fashion, your prescription lenses are doing the same thing. Prescription lens strengths are altered by the degree (or power) at which the lens is convex or concave. This is only one half of how a prescription lens works however. In order for the lens to be as effective as possible in correcting your vision, your pupils must naturally sit with the centre of the lens when looking straight on.
How To Find Your PD
Finding your PD doesn't require any special machinery, but does require skill to be measured accurately. You might remember your optician holding a ruler to your face to gauge your pupil distance. Doing this yourself can be challenging. Luckily your smartphone can now measure your PD for you.
For iPhone Users
For iPhone users who have an iPhone X or newer, you can use Eyemeasure by Dotty for free here.
When using this app, we will ask for a screenshot of the results projected on your face when you are not
wearing glasses, and for the heights projected when you are.
If your iPhone doesn't have FaceID, then you can download GlassesOn using this link.
This version works much the same as the Android version, and is what your optician will likely have asked you to do if they measure your PD using a tablet rather than a ruler.
For Android Users