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Checking Calibration

The lens calibration process is not perfect and sometimes requires the lens data to be massaged.  After completing a lens calibration, verify calibration using Previzion.

  1. Clear scene and point camera to Previzion Default scene.  The checkerboard patterns help see any discrepancies.
  2. Setup your camera with the same settings as the calibration process (same sensor width, trackers, etc.)
  3. Load you new lens file.
  4. Adjust focus to the various focus distances that were calibrated to (min focus, 100cm, 200cm, 300cm, etc.)
  5. Set the key so that the CG scene is visible along side the live action image.
  6. Make sure that your Video Plane Depth is set to a really small value like 10cm to keep the live action image on top of the CG scene.
  7. Slowly zoom from wide to telephoto, paying close attention to the match between CG and live action.
  8. Another approach is to set your focal length and focus in and out.
  9. Write down any lens distances and positions that may need alterations.

The lens data is written as a regular XML file format and should be easily editable, but we recommend using LensGrapher which was designed and built solely for editing and massaging lens calibration data.

We distribute LensGrapher for free to all our Previzion users.  Download it from our Downloads page.

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Lens Data Fixes

LensGrapher can be a really handy tool for editing and visually seeing the bumps in a lens calibration.  The next chapter is dedicated to explaining all tools and concepts in LensGrapher.

The ultimate judge of a lens calibration is not the graphs themselves, but loading the corrected lens file in Previzion, zooming in and out and focusing in and out.