(Edit - just cross-posted with your last couple of posts!)

Hmmm, that's odd. I'm definitely getting just over 12.7mm. (I'm using Poser 9 at the moment and the dolly camera - orbiting cameras have an inherent offset value which nobody seemed to be able to fully explain or give a precise value for - http://www.renderosity.com/mod/forumpro/?thread_id=2880985 )

I got my latest 12.7mm figure using this very simple empirical test setup, but that doesn't seem to be accurate enough*:

• An imported cube OBJ centred at the origin, with inward facing normals, and each face a different colour. (Imported into Poser with all checkboxes cleared so it loads as intended)

• Dolly camera positioned at the origin (DollyX/Y/Z set to zero, all Scale values set to 100%)

• One point light also at the origin (just to make things visible!)

• Render dimensions set to equal width/height.

Set the focal length to my test value.

Set roll/pitch/yaw to 0/0/0 and render. Repeat with 0/0/90, 0/0/180, and 0/90/0 as a quadruple-check.

(*The reason for this question is that I want to be able to stitch a set of six square renders into a vertical cross cubemap without any seam problems)

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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