Why Your PD Reading Changes Between Photos (and How to Stop It)

Last updated: October 2026 | Reading time: 6 minutes

Short answer: your pupils do not move that much between photos; your setup does. The usual causes are distance to the camera, head angle, where you are looking, how the card sits, and lighting. Fix the setup and the readings converge.

The usual suspects

1. Looking at the screen instead of the lens

Your eyes turn inward when you focus close. If one photo looks at the lens and the next at the screen, the apparent gap changes. Always look at the lens.

2. A different head angle

A small turn (even 3° to 5°) or a tilt changes the projected gap. Square your shoulders and keep the lens at eye level.

3. A different distance

Moving the phone changes how big the card and your face look, and changes how much your eyes converge. Rest your elbows on a table to stay at the same distance.

4. The card shifts

Raising your eyebrows or relaxing your hand can tilt the card and change the apparent width. Keep it flat against your forehead with all four corners visible.

5. Lighting and glare

Glare on the pupils moves the detected centre. Soft light from the front or side keeps the iris edge visible.

6. Blinks and half-open eyes

The eyelids hide the top of the iris and make the circle fit harder. Take a photo right after a full blink.

7. Normal natural variation

Pupil size changes with light and the pupil centre can move slightly as it changes. Don't chase differences of a few tenths of a millimetre.

A repeatable routine

1. Phone on a stack of books or elbows on a table, lens at eye level, about 40 cm away.
2. Card flat, all corners visible.
3. Head level, look at the lens, relax your forehead.
4. Take five photos.
5. Ignore the highest and lowest, and look at what remains.

The tool already discards outliers when you take several captures, but it can't repair an unstable setup.

When the numbers still disagree

If three clean photos still differ by more than 1.5 mm, try another room or another time of day, or compare with a different method. See check your PD measurement is right. For anything important, have an optician measure you.

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PD tool fixes · PD measurement mistakes · Measure PD with a phone camera