Shimming Guide

Turn a focus-chart check into a shim recommendation: enter the measured distance and what the scale claims, get the flange error in microns.

0Flange errorFlange distance is spot onEstimated flange-focal-distance offset from your focus check.
No shimming neededCorrectionWithin calibration toleranceErrors under 0.01 mm are calibration-grade — measurement noise dominates.
52.000 mmEffective flange distancePL nominal 52.00 mmAssumes the lens’s own scale is calibrated — a mis-marked barrel shifts this estimate.

This is a thin-lens estimate. Shim, then repeat the focus check — one or two iterations usually land inside tolerance.

LensFlangeNominal sensorYour sensornominal FFDaligned
Offset exaggerated for visibility — not to scale.
▸How this works

Every mount is built to hold the lens an exact distance from the sensor — the flange focal distance (PL: 52.00 mm). Wear, adapters, and rehousings push it off by hundredths of a millimetre, and the focus scale stops telling the truth: if the flange is too long the scale reads far and wide lenses miss infinity; too short and the lens focuses past the marks. Shims — thin precision spacers under the mount — bring it back. Adding shims moves the optics away from the sensor, removing them brings the optics closer.

The math runs the thin-lens equation on both distances (measured from the sensor plane): the flange error is the difference between the image distance for where the chart really is and for where the scale claims it is — δ = i(D) − i(S). It’s an estimate: internal-focus designs deviate from the thin-lens model, a mis-calibrated barrel biases the result, and B4/ENG zooms adjust back focus with a ring instead of shims. Non-adjustable cine bodies are specced to about ±0.02 mm, so don’t chase errors below that unless the shot demands it.