Shooting Guide for Lens Calibration Grids

If you're going to be shooting lens grids for a project (all anamorphic projects!), here are some tips to make it worth the time. You may not be able to follow these tips due to practical considerations, but the closer you are able to follow them, the better your grids will be, and the smoother your tracking and effects work is likely to go.

  • Always use a perfectly regular array of small dots or a checkerboard. See our online grid generator below. Hint: dots are better.
  • Don't have inset “higher resolution” sections: keep the pattern the same.
  • Use many dots or checkers to maximize accuracy, especially on high-resolution cameras and with anamorphic lenses. Only the dot center or checkerboard corner locations provide useful information, not white space or edges.
  • Always print grids on a large-format printer. Never paste/tape/glue pages together.
  • Make sure the grid is flat. Rigid canvas frames may be flat—truly planar. Foam-core and cardboard are not flat. Walls and glass doors aren't necessarily flat. Consider printing on metal with vendors such as WhiteWall.
  • Alternatively, display the grid on a large flatscreen 4K TV monitor.
  • Measure width and height and length of both diagonals of the printed grid (or monitor) to check the aspect ratio and watch for any skew in printing.
  • Always have the grid extend to the edge of the image! (Fisheyes lenses too!) All the useful information is towards the edge of the image. Pixels past the edge of the grid will often be unusable in production shots.
  • Keep the camera axis perpendicular to the plane of the grid. You can assess this by:
    • Comparing the left and right edges of the image, and the top and bottom edges of the image.
    • If in-camera image mirroring is available, flip mirroring (top/bottom, left/right) off and on and adjust until the image is the same both ways. This is easiest to assess with dot arrays, not checkerboards. Less conveniently, do the mirroring checks with Photoshop or SynthEyes or other convenient app.
    • If the backing is very rigid, mount a mirror flush to it, and aim the camera until the image of the lens is precisely centered. If the grid is displayed on a monitor, suitable lighting on the camera will allow the monitor to act as a mirror!
  • Provide even lighting over the grid.
  • Use the same camera settings as the live shoot: HD vs 4K vs other resolutions, movie vs photo mode, etc.
  • Use the same lens settings (focus distance, iris, and zoom) as the live shoot. Shoot some different combinations if there are large changes.
  • Always record (in cm or inches) the dot-dot spacing or size of the checkerboard squares, and the distance along the camera's optic axis from the center of the camera lens's entrance pupil to the grid. These are used by Rectify Grid's Tilt Correction, and may be used (in Synthia, for example) to compute the field of view as 2*atan(0.5*visible_grid_width / distance).
  • Focus! Shoot the grid at your desired shooting distance. Anamorphic lenses are particularly prone to focus-distance effects. This requires that your grid is large enough to cover the field of view. Defocused grids will degrade accuracy and may prevent automated tools from detecting the dots or corners, requiring manual fixups.
  • Catch-22! The wide field of view of anamorphic setups typically means a short camera-to-grid distance, causing the severest focus breathing problems (zoom and focus changes may be 10%), and therefore the least accurate grid for more normal shooting distances. Bigger grids help minimize that, but keep them flat!
  • Shoot a short rack-focus movie where the focus distance is smoothly ramped (eg from infinite to closest) via a remote control (not by hand, which will typically jiggle the camera). Rack-focus sequences may facilitate calibrating focus breathing, especially for anamorphic lenses.
  • Shoot a rolling-shutter calibration sequence: with the camera on a tripod at its nodal point and a short shutter time, shoot a fast-moving left to right and back sweep, while looking at a static feature-rich environment that will produce many trackers over its entire height. Solve the shot in Tripod mode to determine an accurate rolling-shutter value, eliminating it as a confounding unknown on later more difficult production shots.
  • Retain the original camera output files and any sidecar files produced by the camera. These may contain metadata that will help determine camera settings, especially focus distance curves, during the lens grid shoot. SynthEyes can import the focus channel from available shot metadata or from a YAML file.
  • If you must “standardize” camera images (such as by resampling, cropping, or padding them) for post-production, retain the originals (or extract and retain all metadata from them, even if it doesn't appear immediately useful), and do not superimpose additional markings (framing boxes, center marks, etc) over the grid images, which might make processing the grids automatically more difficult, time-consuming, or less accurate. Since lens grids may contain a few thousand trackers, causing rework can blow up costs rapidly.
  • Apply the same image standardizations to the production images that you performed to the lens grids—otherwise the lens grids are likely to be unusable.
  • Retain all metadata from live shoots!
  • Check back to this page for possible future tips!
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