I've just completed another test. Here's a section of my resultant 8192x4098 lat-long map, created from separate 90° 2048x2048 cubemap renders. It shows the seam between the 0° yaw and -90° yaw renders. The version on the left was created from dollycam renders with a 12.7298mm focal length (bagginsbill's figure), and on the left with 12.7mm (millighost's figure).

I was rather surprised - I can see the seam in the 12.7298mm result, but not in the 12.7mm one.

The result's not conclusive yet (I may have made a mistake somewhere - I was sure I'd tried 12.7 before and could spot the seams!) and I still need to do a few more tests.

aRtBee: I've got your pages bookmarked, as there's lots of interesting and new (to me) stuff there. I particularly liked your comment on the orbiting cameras - "This is very satisfying from a computational point of view, but very confusing for us, humble human users."  Well said!  :D

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The 3Dcheapskate (also available in DAZ and HiveWire3D flavours) occasionally posts sensible stuff. Usually by accident.
And it usually uses Poser 11, with units set to inches. Except when it's using Poser 6 or PP2014, or when its units are set to PNU.