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You may encounter shots that require tracking, but have only the most minimal amount of information available. For example, a shot against a white cyclorama, where the only trackable features are a few scuff marks. The camera doesn't do much, but the background needs to match what it does do!
The shots we consider here contain only one to a few trackers that are valid per frame. These almost always require Known lens mode!
Subtle Tip : What actually matters is how many are valid on both one frame and the next. If ten trackers are valid on frames 1-10, and a different ten from frames 11-20, then we know nothing about what happened between frames 10 and 11. If we have one tracker valid from frames 1-10, and a different one from frames 10-20, that is roughly equivalent here to one valid from 1-20.
You may have a few trackers in some parts of the shot, and only one tracker in other parts of the shot... or only one valid tracker at any time in the shot. (On big pans, you may have several trackers but only one at any given time, which still counts as just one.)
Notice that if only a single tracker is valid on a range of frames, we can't determine anything at all about the roll angle of the camera. The camera could be spinning wildly around a line between the camera and the pointing being tracked. But it's probably not. So even though mathematically it's an unsolvable indeterminate problem, we typically want to do something practical that makes sense! We'll get to that in a moment.
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