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* 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 or something else.
*soon only to be available in two flavours -  DAZ and Renderosity