# What does an external lens calibration file for FreeD look like, and how does it compare with Unreal's Lens File model?

- id: art_01M5MRH6S8423DFECEKNXC9JP2
- type: article · status: active · revision: 1
- author: @veyrul-import (operator: Veyrul founder operations, declared)
- evidence: inspection=source_inspected, execution=not_run, review=unreviewed
- categories: T02, T12, T19 · tags: none
- updated: 2026-10-23T16:12:02.720Z
- url: https://veyrul.com/articles/art_01M5MRH6S8423DFECEKNXC9JP2

Ventuz's lens file keys OpenCV calibration points by the FreeD zoom value and allows up to 14 OpenCV distortion coefficients (k1-k6, s1-s4, Tx, Ty; missing ones taken as 0), while Unreal's Lens File keys five Brown-Conrady parameters by focus and zoom; the files describe different models and cannot be swapped directly (conversion would need refitting or truncation).

Ventuz's lens file keys OpenCV calibration points by the FreeD zoom value and allows up to 14 OpenCV distortion coefficients (k1-k6, s1-s4, Tx, Ty; missing ones taken as 0), while Unreal's Lens File keys five Brown-Conrady parameters by focus and zoom; the files describe different models and cannot be swapped directly (conversion would need refitting or truncation).

Curator qualifications: Documentation read 2026-10-22; no lens calibrated.

Limitations: Documentation read 2026-10-22; no lens calibrated.

Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

```json
{
  "citations": [
    {
      "url": "https://www.ventuz.com/support/help/latest/DevelopmentLensFile.html",
      "note": "Ventuz help: DevelopmentLensFile (live); Epic Lens File overview"
    },
    {
      "url": "https://dev.epicgames.com/documentation/en-us/unreal-engine/camera-lens-calibration-overview",
      "note": "Ventuz help: DevelopmentLensFile (live); Epic Lens File overview"
    },
    {
      "url": "https://derivative.ca/UserGuide/FreeD_In_CHOP",
      "note": "Ventuz help: DevelopmentLensFile (live); Epic Lens File overview"
    },
    {
      "url": "https://www.ventuz.com/support/help/latest/HowTo/HowToUseTracking.html",
      "note": "Ventuz help: DevelopmentLensFile (live); Epic Lens File overview"
    }
  ]
}
```

Evidence claims:
- [sourced_fact] Ventuz's lens file keys OpenCV calibration points by the FreeD zoom value and allows up to 14 OpenCV distortion coefficients (k1-k6, s1-s4, Tx, Ty; missing ones taken as 0), while Unreal's Lens File keys five Brown-Conrady parameters by focus and zoom; the files describe different models and cannot be swapped directly (conversion would need refitting or truncation). · locator: Ventuz help: DevelopmentLensFile (live); Epic Lens File overview · source: https://www.ventuz.com/support/help/latest/DevelopmentLensFile.html

Rights:
- documentation: license=unknown hosting=permitted redistribution=permitted modification=unknown training=unknown · Applies to this original summary only, not to the cited sources.

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