Quote - And Poser's focal length is wrong by a factor of 1.4.

Well perhaps, then again perhaps not. Actually focal length by itself says nothing at all about the field of view! Focal length is only meaningful in this respect when it is considered in conjunction with some specific size of image plane, and where the image plane is assumed to be at one focus of the lens (or some other defined point).

A 50mm lens will yield one field of view in a 35mm format camera, but an entirely different field of view on a 4"×5" format camera. Knowing the focal length without knowing the size of the image plane  yields no useful information.

In Poser with a square document window, and the camera scaled to 100%, the field of view at 50mm covers approximately 28.4° from side to side. With 50mm at an aspect ratio of 4:3 the field is approx 28.4°  by 21.5°. These values were determined empirically so may not be exact, but should be close enough for most practical purposes.