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Measuring Lens Nodal Point in the Field

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Accurate nodal point measurement is essential for target tracking, probe/snorkel lens work, and any move where the camera rotates around a point without parallax shift. This guide covers a practical field technique using two reference objects and basic trigonometry — no specialist equipment required.

In plain language, this measurement tells Flair how far the lens's viewing point sits in front of the robot's pan/tilt pivot. When the number is correct, a nearby object and distant object remain lined up as the camera pans. Parallax is the visible sliding of those objects against each other when the rotation point is wrong.

What is the Nodal Point

The front nodal point (also called nodal distance or tube length in Flair) is the distance from the pan/tilt pivot centre of the rig to the lens' no-parallax point. When the camera is rotated around this point, foreground and background objects do not shift relative to each other.

In Flair, this is set as the Tube Length (or Nodal Offset) in the camera setup, expressed in metres.

Field Measurement Procedure

Equipment

  • Two thin vertical objects at different distances (e.g. two lighting stands, Allen keys on a ladder)
  • Track and pan axis working in Flair

Steps

  1. Set Rotate to 0. Position the rig so pan is perpendicular to the track (90° pan angle).

  2. Align two reference objects in frame centre. Place two objects at different distances (one close, one further) so they are perfectly aligned in the centre of frame at this position.

  3. Store this position in Flair (as a Goto position or note the exact axis values).

  4. Move only Track and Pan until the two objects are aligned at the frame edge (e.g. right edge). The foreground and background objects should align at the edge of frame.

  5. Store this second position (or note Track and Pan values).

  6. Calculate the nodal distance. Use a calculator with a sine function. If it offers degrees/radians, select degrees and use the pan difference directly. The formula below assumes the calculator receives the angle correctly:

Nodal Distance = Track difference ÷ SIN(Pan difference)

Where: - Track difference = Track position at step 3 minus Track position at step 1 (in cm) - Pan difference = Pan angle at step 3 minus Pan angle at step 1. Use degrees mode, or convert the angle to radians if the calculator is in radians mode. - Result is nodal distance in cm (divide by 100 for Flair metres)

"Rotate at 0, pan perpendicular to rail (90°); align two lighting stands in centre. Store position; move only Track and Pan until stands align at frame edge. Track diff ÷ SIN(pan diff) = nodal distance in cm." — Dig (Digby Strange)

Tips

  • Use Allen keys on a ladder for precision — they are thin, vertical, and easy to centre in frame (tip from Heiko Matting).
  • Check the result in Carts mode — set the nodal offset in camera setup and verify that rotating pan produces no parallax between the two reference objects.
  • Works for zooms at multiple focal lengths — repeat at each focal length you need calibrated.
  • For anamorphics — measure both pan nodal (horizontal) and tilt nodal (vertical) separately, as they may differ.

Entering the Value in Flair

  1. Go to Setups → Kinematics Setup (or Camera Setup depending on Flair version).
  2. Find Tube Length or Nodal Offset — enter the measured value in metres.
  3. Apply.
  4. Test by rotating pan through a wide arc and confirming foreground/background alignment is stable.

The Other Offset Fields (Camera Mount Setup)

When Flair's Camera Mount feature is used (see Camera Mount vs traditional Lens Setup, above), the Front Nodal Offset sits alongside several related fields — all measured relative to the camera's own sensor/film plane, not the robot joint:

  • Front Nodal Offset — as measured above; the lens's optical no-parallax point, as a distance from the sensor/film plane (e.g. 0.12 m for a 12 cm measurement).
  • Side Nodal Offset and Vertical Offset — normally left at zero. Only needed for an unusual lens where the optical axis is not centred on the mount.
  • Measure Offset — only needed if you measured your calibration distance from somewhere other than the sensor/film plane itself (e.g. from the front of the lens). Leave at zero if you measured directly from the sensor. Sign convention: positive when the measurement point is in front of the sensor (towards the lens); negative if measuring from behind the camera body.
  • Focus Assist Offset — only relevant with a Focus Assist depth sensor module fitted; enter the distance from the sensor/film plane to the module.

Why this matters: with Camera Mount enabled, all of the above stay relative to the camera body — if the camera later shifts along its mounting plate, you only need to update the Camera Mount offset once and every lens calibration remains valid. Without Camera Mount, the equivalent X/Y/Z Nodal Offset fields live directly in Lens Setup and are relative to the robot's tilt-axis joint instead, meaning a camera shift requires re-entering the offset for every lens individually.

YouTube 07 - Lens Calibration Part 2 Flair Classic

▶ 04:00 — X/Y/Z Nodal Offset and Measure Offset chapters: entering measured nodal values into Flair's lens setup

YouTube Camera Mounts & Offsets Flair 7

▶ 00:18 — Front Nodal Offset: what it is and how to measure it (12cm example) ▶ 02:08 — Measure Offset, Focus Assist Offset, and why these fields are relative to the sensor when using Camera Mount ▶ 03:25 — Without Camera Mount: the same offsets move into Lens Setup as X/Y/Z Nodal Offset, relative to the robot joint instead