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Solving for Lens Breathing
As described, lens breathing results in a zoom, even if it is a prime anamorphic lens. For anamorphic lenses, the aspect ratio can change as well, which we solve for as a change in vertical scaling (VSCL).
If we allow arbitrary changes to the vertical scaling, it is not only possible but likely for simple shots to change aspect or even squish down to nothing, a narrow horizontal strip. Without motion in the shot to prevent it (such as rare rotation around the axis of the lens), you need to prevent that as follows.
Select several trackers distributed across the image, such as one good one in each corner, and on the coordinates panel, click Set Seed, then change them to Lock Points. This will “lock in” the already-computed aspect ratio of those points.
Next, change VSCL to “Animate at keys” parameter solving mode. In severe situations, you may also need to put the calculated distortion parameters
into Animate at keys mode also, so that the distortion can change in addition to the scaling.
Then, use the +Keys buttons to place key frames at the beginning and end of the transition, and other locations within it depending on the shape of the transition. For a smooth transition you might want a key or two at the beginning and end, and maybe one in the middle; for a long slow tracking shot, you may want to put keys every 5 or ten frames (see the “Lens/Add distortion keys” script). (The +Keys button will put keys on all parameters that are in Animate at keys mode.)
If you don’t have any idea where a focus pull might be in the shot, for shorter shots you can add keys throughout the shot every few frames. You can use the “Script/Lens/Add distortion keys” script to place keys every n frames over the current playback range, on animate-on-keys parameters.
Reminder: The “Metadata/Load focus channel from metadata” script can extract any specific metadata item and put it on the display-only “focus” track for the current camera in in the Graph Editor. Or, use the “File/Import/Metadata/Import focus channel from YAML” importer. That focus curve will tell you where the pull is located and its approximate shape. It’s a good idea to preserve metadata from the original camera recording!
If you don’t know where the pulls might be in a long shot, you can set up VSCL for Animate by frame mode, which will solve for those parameters on every frame. Although this sounds like a great idea, one and done, instead it is a terrible idea that will at best produce a very noisy solve, and at worst destabilize it totally.
IMPORTANT: Shots using Animate by frame mode should almost never be released. Instead, use this mode to identify where the pulls are, then delete most of the keys and switch to Animate at keys mode for a smooth solution.
After solving the shot in Animate by frame mode, remove excess keys and switch to Animate at keys mode and refine the solve. You can use the “Lens/Decimate lens distortion keys” script to do that; you should follow up with a Refine solve.
As always, use canary zero-weighted-trackers (ZWTs) to look for over-
fitting.
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