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Zoom Linearise and Contra Zoom — Field Guide

CONFIRMED ANY RIG WITH ZOOM

Summary

Zoom Linearise maps the zoom motor's raw encoder position to actual focal lengths, creating a linear relationship between motor position and the lens's optical properties. Once linearised, Flair can run smooth, mathematically predictable zoom moves — including contra zooms (dolly zoom / Hitchcock zoom), where the camera physically moves while the zoom is adjusted to keep subject size constant.

This page documents the step-by-step field procedure from Dig's shared guide ("HOW TO USE ZOOM LINEARISE OR CONTRA ZOOM IN FLAIR.docx", 2021-09-22).

When to Use This

  • You want smooth, accurate moves in "zoom in focal lengths" mode.
  • You want to run a contra zoom (dolly zoom / pull back while zooming in, or push in while zooming out).
  • The zoom motor's encoder scale does not feel linear — a constant encoder change at the wide end produces a different apparent zoom speed than the same change at the long end.

Part 1 — Zoom Linearise Setup

Step 1 — Physically Move the Zoom to the Widest Focal Length

With the zoom motor disabled (motor off), manually rotate the lens to its widest focal length. This is your zero reference point.

Zero at widest focal length, not end of travel

The zero point must be the widest focal length of the lens, not the physical end of motor travel. Using end of travel as zero produces incorrect focal length mapping.

Step 2 — Set Up the Zoom Axis

In Flair, open the zoom axis setup. Confirm the axis numbers increase in the correct direction:

  • Axis values must increase as the zoom moves toward the telephoto end.
  • If the values decrease as you zoom in (telephoto), reverse the motor direction in the axis setup.
  • Axis values must be positive throughout the zoom range — they cannot go negative.

Maintain the correct backlash direction throughout calibration. Always approach calibration points from the same direction (whichever direction removes backlash from the zoom gearing).

Step 3 — Enable "Zoom in Focal Lengths"

In Flair, enable "Zoom in Focal Lengths" mode (found in the axis/lens calibration settings). This switches the zoom display and move programming from raw encoder units to actual focal length values.

Step 4 — Enter Calibration Points

Navigate to the Zoom Linearise calibration table. Add calibration rows by:

  1. Moving the zoom to a known focal length marking on the lens.
  2. Entering the focal length value (e.g., 24mm, 50mm, 100mm) in the table.
  3. Repeating for as many focal length markings as the lens has.

More calibration points = more accurate linearisation. Use at least the ends of the zoom range plus any intermediate marked focal lengths.

Step 5 — Calculate and Apply

After entering all calibration points, click Calculate, then Apply. Flair generates the linearised mapping.

Test by setting a zoom move from one focal length to another — the apparent zoom rate should now feel consistent across the zoom range.

Part 2 — Contra Zoom (Dolly Zoom / Hitchcock Zoom)

A contra zoom keeps the subject the same apparent size in frame while the background perspective changes. The camera physically moves toward or away from the subject while the zoom adjusts to compensate.

Requirements

  • Zoom Linearise must be completed first (above).
  • A track or dolly axis is required to physically move the camera.
  • The move should be a straight A-to-B move — curves or multipoint moves give poorer results with contra zoom.

Tips for Best Results

  • Wider starting focal length — a wider zoom range gives the effect more visual impact.
  • More subject-background separation — a greater distance between subject and background amplifies the perspective distortion effect.
  • The effect works best when the subject is at a significant distance from the background.

Programming the Contra Zoom Move

  1. Program the dolly/track move as a simple A-to-B move.
  2. At the same keyframes, set the zoom to compensate: if the camera moves back (increases distance to subject), zoom in to maintain subject size; if the camera moves in, zoom out.
  3. Use the focal length values to calculate the correct zoom compensation — at each keyframe, the zoom should be set so the subject remains the same size.

Straight moves only

Curves or multi-point moves produce inferior contra zoom results. Use a straight A-to-B dolly move for the best effect.

"Zero at widest focal length, not end of travel. Axis values must increase and be positive — reverse motor direction if not. Use zoom in focal lengths. Contra zoom works best as straight A-B move." — Dig (field guide, 2021-09-22)

Official Documentation

Flair Classic Manual — Ch.5 Setups: Lens Setup — p.90

Flair 7 Manual — Ch.5 Setups: Lens Setup — p.52