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Broadcast Lens Calibration (Fujinon / Canon with Internal Motors)

BOTH

Fujinon and Canon broadcast lenses with internal motors and their own encoders require a different calibration approach from standard cine lenses driven by external MRMC lens motors. The internal motor has a fixed scale factor and its own "zero" position — which is not necessarily at infinity — and the lens travel typically extends past infinity.

Key Differences from Standard Lens Calibration

With a standard cine lens driven by an MRMC motor:

  • You zero the encoder at infinity.
  • You calibrate scale and position using normal procedure.

With a broadcast lens (Fujinon / Canon internal motor):

  • The lens has its own internal encoder with its own scale (e.g. 0 to 10, with scale of 0.1 or 0.15).
  • The zero position is not at infinity — it is somewhere in the lens travel.
  • The travel goes past infinity.
  • You must calibrate backwards from the normal procedure.

"These broadcast lenses however, travel usually from 0 to 10, with that fixed scale, and the 0 is not necessarily on infinity. So we actually have to calibrate these backwards." — @peter

Calibration Procedure

Step 1 — Back focus (collimation)

  1. Set the zoom to fully telephoto (zoom right in).
  2. Focus on something as distant as possible.
  3. Adjust the back focus (collimation) adjustment of the lens so that as you zoom fully in and out, the focus does not change.

Back focus quality

The success of this step depends on the lens quality and the adjustment range available. Some older broadcast lenses have limited back focus adjustment.

Step 2 — Choose a fixed focal length

Set the zoom to a fixed position in the middle of its travel, on a zoom mark. Use that focal length for all subsequent calculations and calibration. Do not change the zoom during calibration.

Step 3 — Find and mark infinity

  1. Point the camera at something at optical infinity. If in a studio, open a window or door and look up the street. Find a point that is definitively at infinity.
  2. The lens's own infinity mark will be imprecise — mark the true infinity position with tape on the barrel.

Step 4 — Read the encoder at infinity and store the offset

  1. Read the encoder value from the lens at the infinity position (e.g. the value might be 1.5 or -0.86).
  2. Store this value as the Infinity Offset in Flair's lens setup. If it is negative, store it as negative.

Step 5 — Calibrate using the lens's own zero position

Calibrate the lens as normal, but use the lens's own encoder zero as your starting reference rather than infinity. The internal scale will be calculated as part of this calibration and set as the scale factor in Flair.

Step 6 — Iterate if needed

If the calibration nearly works but drifts between the three positions (infinity, mid, close):

  • The focal length is probably slightly incorrect.
  • Adjust the focal length value in Flair and recalibrate.
  • Alternatively, use the Lookup Table calibration method for broadcast lenses.

Repeat the calibration as many times as needed — each iteration brings the scale factor closer to the correct value.

"You may have to go round and repeat this over, using the infinity mark you found. The more times you do it with Flair the closer you will get to correct, once their scale figure has been approached." — @peter

Lookup Table Method

If the 3-point calibration does not converge accurately (common with broadcast lenses whose scale is irregular), use the Lookup Table calibration instead. This maps individual encoder positions to known focus distances without relying on a single scale factor.

See Focus Calibration — 3-Point vs Lookup Table for when to use each method.

Source: Fujinon/Canon Lenses with internal Motors (Broadcast lens) — talk.mrmoco.com