Short answer: the PSA 10 line on a standard border is a 0.25 mm offset. A ruler or centering tool with 0.5 mm marks cannot resolve it; drawing lines on a photo by hand drifts by 0.3 mm or more with tilt and lens distortion. Only automatic edge detection with perspective and lens correction gets to ±0.1 mm, which is what the line needs.
Centering Cam: 95% of measurements within ±0.11 mm on a 40-photo ground-truth test. Shoot, read the mm margins, see seven grader verdicts.
55/45 means the difference between the two margins may be at most 10% of their sum. On a 2.5 mm border (sum 5 mm) that is 0.5 mm, so the print may be 0.25 mm off centre. A 0.5 mm ruler tells you "2.5 or 3.0", not "2.75 versus 2.25" — and that pair is exactly the line.
Where hand-drawn lines go wrong
Tilt. Five degrees off vertical changes the apparent top/bottom margins by 1–2%; fifteen degrees approaches 0.3 mm on a 2.5 mm border.
Line placement. Outer edge of the frame or its centre? That alone is 0.1–0.2 mm, and differs from person to person.
Lens distortion. Phone wide-angle lenses bend straight lines near the frame edge; a card placed off-centre gains width on one side.
What automatic detection does
Finds all four cut edges as straight lines and rectifies the trapezoid back to a true rectangle.
Estimates the phone's lens distortion per device and corrects it (self-calibrates after a few shots).
Finds the printed frame on all four sides and reports edge-to-frame distance in mm, flagging shadows, glare and blur.
For cards without a frame, aligns the artwork to a reference template and measures the print offset instead.
Shooting the same card three times yields a median and a range; results that straddle the limit are held for a reshoot rather than guessed.
FAQ
Can I measure centering with a ruler?
Roughly. The PSA 10 line on a standard border is a 0.25 mm offset, below what 0.5 mm marks can resolve.
Are line-drawing centering apps accurate?
They depend on hand placement and camera tilt and typically drift by 0.3 mm or more. Fine for listing photos, not for a grading decision.
How accurate is Centering Cam?
95% of measurements within ±0.11 mm on a 40-photo test with known offsets. Accuracy depends on shooting flat, straight above and out of the sleeve.