One of the optometric terms that has become rather popular in the past few years is pupillary distance. This has been held up as a holy grail by people who want to order their glasses online instead of going into an optometrist's office because it's not always required to be on the prescription copy that you get for your records. Yet it's an essential part of fitting glasses correctly.
What It Sounds Like
The term "pupillary distance" is straightforward: It's the distance between your pupils. The measurement is taken from the center of one pupil to the center of the other. The measurement is in millimeters, making this a very tricky measurement to take yourself. It's possible to do it, but the small size of a millimeter means that you can easily be off by one or two.
The Sweet Spot
Pupillary distance is needed mainly to ensure that you get the most out of your prescription. Because of the shape of a lens, there's a sweet spot in the center of the lens that offers the best sight. The lens curves around to accommodate for your side vision, so of course you'll still be able to see through the lens even if you're not looking dead center. But the center spot is the best.
Knowing the pupillary distance allows the optical lab making your lenses and frames to make a pair of frames and lenses that place the sweet spot in front of your pupils. If the pupillary distance is wrong, then you might find your sight when you look straight ahead to be too blurry.
Getting the Look Right
Knowing the pupillary distance also helps ensure that the frames aren't too big or too small. While your overall head size is a main determining factor there, your eyes also play a role because your eyes could be wide-set, narrow-set, slightly off-kilter, and so on. If the lab knows where the center of your pupils are, the staff can determine what size of frames will be right for you. If you have frames that are too big or too small, the lenses will be off-center compared to your eyes, and you won't be able to see that clearly because, again, your eyes and the sweet spot on the lenses won't match up.
If you buy your glasses from an independent optical lab, you should still try to get your pupillary distance from an optometrist. This measurement will be more likely to be accurate, which only helps the lab create a great pair of glasses for you.
For an optical lab, contact a company such as Identity Optical.