I don't think you can actually set the focalength in Bryce but you can set the field of view numerically (FOV). The default 60 degrees in Bryce is a bit on the wide side, appx that of a moderate wide-angle lens. The field of view of a normal lens (in 35mm photography) is 46 degrees for a 50mm standard lens. I have (had) info on fields of view for various focal length lenses, but can't get my hands on it right now...