Optical measurement

Pupil/Seg Height Precision Distance for Loupes & Glasses

Your PD is the distance between the centres of your pupils. Use a credit-card-sized object as a known reference and read your measurement in millimetres — everything stays on your device.

Cartoon example of a person holding a bank card horizontally under their nose, facing the camera — PD: hold the card flat under your nose, face the camera straight on.

Diagram of a face wearing glasses with a vertical arrow measuring seg height from the pupil centre down to the bottom rim of the lens — Seg height: pupil centre down to the bottom inner edge of the lens — used for progressive corridors and loupe barrel placement.

Hold a standard card (credit / ID) flat against your forehead or under your nose, then take a straight-on photo.

Why 85.6 mm?

Every ID-1 card — credit, debit, driving licence — is exactly 85.6 mm wide, making it a reliable ruler.

Typical range

Adults usually measure 54–74 mm. Children run smaller. Anything far outside means a marker is misplaced.

Not a prescription

This is a good-enough estimate for single-vision frames. For progressives, ask an optician to measure.