Your fStop is quite shallow, which accounts for the softness of the render, especially with a head-and-shoulders portrait.  The general rule for portrait photography is to focus on the eyes and give yourself a generous fStop, maybe around f8, to achieve some overall sharpness on the face.  At least, that's my target aperture value, I might push it f11 depending on the focal length of the lens in use.  Although, Poser doesn't follow photography conventions (too bad), the longer the focal length of the lens, the shallower the DoF at lower fStop values.