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sk66 wrote:
No, 3D is horizontal, vertical (and the diagonal combination), plus distance. E.g. a 2D image vs a 3D reality... I linked Nikon's own definition/explanation of the difference.
There is nothing special at all about Nikon's 3D tracking in terms of it's ability to adjust for distance changes compared to Nikon's other focus modes - they all work in exactly the same way when it comes to making distance adjustments via PDAF. All AF-C modes are also inherently predictive with or without 3D tracking.
All 3D tracking is doing is gathering additional information from the subject (like shape and color data from the meter) in an attempt to create a unique profile to track the subject across the frame (and when it re-enters the frame), particularly subjects with random or erratic movement. Originally it worked best with faces and now it works with pretty much anything as higher resolution meters give the camera better subject discretion. The higher resolution the meter, the better it works, which is why it got a performance bump in the D5/D850/D500 DSLRs and of course a massive upgrade with mirrorless.
I'm not sure what you may have read that said 3D tracking is the only AF mode that can deal with distance changes, but that has never been how 3D tracking works within the context of Nikon AF.
The word "distance" isn't used anywhere with regards to 3D AF tracking - not in Nikon's manuals, not in their supplementary technical documentation, and not in the glossary link you provided. I think where you're being confused is that the camera's Matrix metering mode takes into account subject distance information for exposure calculations, but that distance information still comes from the PDAF system & lens, not the meter itself - it is nothing new and that same information is used in the other AF modes. The distance information taken from the lens/PDAF system is taken into account in Matrix metering for a more accurate exposure, it is not new information that is added to improve autofocus when using 3D AF.
Here is Nikon's explanation of 3D AF Tracking from the link you provided - note there is no mention about a distance calculation, because the camera is doing that anyway regardless of AF mode:
An invaluable feature for sports, action and wildlife photography, 3D focus tracking, available in select Nikon D-SLRs, automatically shifts the focus point to follow the movement of the subject. With the shutter release pressed halfway, you'll see in the viewfinder the lens continuously maintain focus as the subject moves.
However, maintaining focus doesn't guarantee a sharp image, as there is a short time lag between the release of the shutter and the capture of the picture. To solve this problem, the focus tracking system is a predictive system that uses special algorithms to forecast the position of the subject at the moment the image is captured. The prediction is based on a measurement of the subject's movement and speed.
Simple predictive tracking is very effective for pictures of a subject moving at constant speed toward the camera, but to provide maximum focusing performance for a subject that abruptly changes direction at high speed, or a subject with low contrast, moving randomly, the AF system must accumulate subject location data using multiple focus areas. The AF modules built into selected Nikon D-SLRs have as many as 51 focus areas that can detect vertical, horizontal and diagonal movement of the subject.
To realize high-precision AF for high-speed continuous shooting of a fast-moving subject, the processing speed of the AF cycle is vital. To provide that speed, a Nikon proprietary technology called overlap servo prepares for the focus detection of the next shot while the lens-driving operation for the current shot is in process.
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If there are no distance changes, the AF system doesn't really have to do any work beyond the initial acquisition (which also requires a distance calculation).