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Backward Multi-Pass Reveals Rolling-Shutter Skew Mismatch vs Forward Pass

Summary

Shooting one pass of a multi-pass move forward and a second pass of the same move backward (intending to composite both, e.g. to show an object from both directions) revealed visible image skew/warp when comparing the two passes on cameras with slow sensor readout, such as the Alexa Mini LF. This is a rolling-shutter artifact, not a rig stability or sync problem — a pan/tilt move that reads out top-to-bottom at different real-world timing depending on direction of travel will show geometrically different skew between the forward and backward pass, even when both are frame-accurately synced.

Symptoms

  • Multi-pass shot with one pass run forward and a second pass of the identical move run backward.
  • Comparing the two passes shows the image appears skewed or warped differently between them, not just mirrored/reversed as expected.
  • Confusing because sync/timing checks show both passes are frame-accurate — the issue is visible in the image itself, not in timing.
  • Most visible on cameras with slower sensor readout (e.g. Alexa Mini LF); a global-shutter or fast-readout camera would not show this artifact.
  • Also observed independently as diagonal pan/tilt skew during fast single-direction moves on Milo with a slow-readout camera — same underlying rolling-shutter cause, not specific to multi-pass work.

Community Guidance

[FIELD GUIDANCE] Rolling Shutter, Not a Sync or Stability Problem

Community — Simon Wakley, 49 179 1011493 — 21 July 2026

Simon Wakley: "I would be VERY surprised if it worked perfectly. There are many variables so I would cover yourself and shoot clean plates both ways. It also depends on the shutter pulse of the camera and if it is at the start of the frame the middle of the frame or the end of the frame."

"I did that and noticed the readout speed... We had a rolling shutter effect in different directions. Looked like the image was skewed. Really weird. I think it was the Alexa mini LF which didn't have a good readout speed. If you do fast moves try to push for a global shutter camera."

"I did that with Milo so it was not stability but a really confusing skew in the image when you compared. Diagonal pan/tilt move... The readout speed of that camera is extremely slow compared to others."

Guidance:

  1. If you need forward and backward passes to composite cleanly, budget for a global-shutter or fast-readout camera rather than assuming any correctly-synced camera will match between directions.
  2. If stuck with a slow-readout rolling-shutter camera, shoot clean plates both directions as a safety net rather than relying on a single pass plus a mirrored/reversed composite.
  3. Be aware this same rolling-shutter skew can show up on a single fast diagonal pan/tilt move, not just in multi-pass comparisons — don't misdiagnose it as a rig or sync fault.
  4. Some post rotoscoping of the reversed-pass element may be needed even with careful shooting.

confidence_score: 0.68