Hi Brian, the 12 has to do with the film size of a 35mm camera, it has a film size of 36x24mm. So that combined with the focal length determines the field of view angle. You use 12 because in MoI the field of view controls the innermost square of the view (http://moi3d.com/forum/index.php?webtag=MOI&msg=3644.34http://moi3d.com/forum/index.php?webtag=MOI&msg=3644.5) so that corresponds to the 24mm height, then to form a right triangle you take half that:
