Measure your PD online with your phone camera

The honest answer is that it depends entirely on the camera. On a recent iPhone, the front camera isn't just taking a picture — the TrueDepth system projects and reads thousands of invisible depth points, so it knows how far each part of your face is from the phone. That depth is what lets the app convert the position of your pupils into a real millimeter PD, typically within about 1 mm of what an optician measures with a pupillometer. It's the same reason you don't need to hold a credit card or coin against your face for scale: the depth data already provides it.

A 2D webcam or a flat selfie is a different story. Without depth, those tools have to assume a scale — usually from a reference card you hold up — and any tilt of the card or your head feeds straight into the error. That's also why a printable ruler in a mirror is so easy to get wrong: you're reading your own eyes against a reflection. For most prescriptions a careful camera measurement is plenty accurate; for strong prescriptions, where a millimeter or two matters more, take two or three readings and average them — and if a result ever disagrees with a number your optician gave you, go with the optician.

Why a camera app beats a web PD form

Most "measure PD online" pages are really just a text box that asks you to enter a PD you already have — they don't measure anything. A camera-based app does the measuring for you on the spot, captures the near and monocular values that those forms usually skip, and keeps the result on your device with no account and nothing uploaded. If you already know your PD, a web form is fine for storing it; if you don't, the app is what actually gets you the number. For a deeper look at the app itself, see the pupillary distance app, and for every method measured by hand, read how to measure pupillary distance.