The PD Journal
PD Measurement Methods Compared: Ruler, App, Optician and Pupillometer
Last updated: October 2026 | Reading time: 8 minutes
Short answer: an optician with a pupillometer is the reference method. A bank-card photo method can be a good estimate for simple glasses. A mirror and ruler is the cheapest but the most dependent on your technique. Choose by how strict your prescription is, not by convenience alone.
At a glance
| Method | Cost | Equipment | Main error sources | Best for |
|---|---|---|---|---|
| Optician with pupillometer | Free to low fee, sometimes included with glasses | Pupillometer | Operator technique, fixation | Progressives, strong or children's lenses |
| Optician with PD ruler | Free to low fee | PD ruler | Parallax, operator technique | Most prescriptions |
| Photo + bank card (our tool) | Free | Phone, card | Card detection, depth offset, head pose, convergence | Basic single-vision glasses |
| Photo + face-landmark app only | Free | Phone | Assumed iris size or camera data | Rough estimate |
| Mirror and ruler | Free | Ruler, mirror | Parallax, convergence, hand movement | A sanity check |
| Existing glasses read on a lensmeter | Low | Optician's lensmeter | Frames may not match your PD | A cross-check |
Optician with a pupillometer
A pupillometer is an instrument you look into; it measures the distance between pupil centres at a set fixation distance. It is considered the standard clinical tool and is the best option for progressives and strong prescriptions. Measuring fitting height as well is normally done at the same visit.
Optician with a PD ruler
A ruler held in front of your eyes, read by someone standing opposite. Skill and parallax matter, but it is widely used.
Photo with a bank card
Our approach. The card gives scale; software finds the pupils; the tool corrects for geometry. It is convenient and cheap, but it depends on getting a clean photo. See how it works. For the independent evidence on phone apps, see the published comparison of smartphone PD apps (PMC10389117).
Mirror and ruler
You measure yourself with a ruler against your brow while looking in a mirror. Parallax (your line of sight is not perpendicular to the ruler) and the inward turn of your eyes both bias the reading, and it is hard to hold still. Useful as a sanity check; see how to measure PD at home.
Which should you choose?
- Simple single-vision glasses, normal prescription: a careful photo measurement is reasonable, ideally repeated and cross-checked.
- Progressive lenses, strong prescriptions, children, prisms: have it measured professionally.
- Unsure? Do both, and see if they agree.
Related
Online PD apps: how accurate? · Check your PD is right · Why your optometrist won't give you your PD