Every optical lens has a “sweet spot” where the prescription is exactly what you want. Because of the physics of grinding an optical lens, the further away you are from the “sweet spot”, the more distortion there is of the prescription. So ideally, the sweet spot is centered over the patient’s pupils. Most people’s faces are pretty symmetric so the PD for the two eyes is usually pretty similar. A plano lens has the same prescription everywhere: the whole thing is a sweet spot, so the PD doesn’t need to be specified. In general, the stronger the prescription is, the smaller the sweet spot is. So someone with just a diopter or two of nearsightedness, farsightedness, or astigmatism, gets pretty good vision over most of the lens. A person who is very nearsighted has a small sweet spot. The stronger your prescription is, the more critical the PD is. This is one of many reasons people with strong prescriptions prefer contact lenses: the lens moves with your eye so you’re always looking through the sweet spot. The PD is also super-important if the lab is grinding a free-form progressive (with an add): progressive lenses already have a fair bit of distortion in them, just by their nature. Instead of having a “sweet spot” you have a “sweet path” through the lens, going from top to bottom. This path is referred to as the “channel” and the higher quality and more expensive progressive lenses tend to have a wider channel to look through. If your prescription does not have PD listed, please contact your provider. They may have it on file, but they sometimes do not measure it if you do not purchase glasses from them.