If you are familiar with photography, the fStop value, aperture value, determines how much light reaches the sensor plate and how much distance is in focus before and behind the your point of focus.  Since lighting is not influenced by fStop in 3D programs, the aperture value range will determine the focal depth distance.  On a camera lens, a large fStop value will make the aperture opening small thus increasing your depth of field and making the image foreground and background appear sharper.  Decreasing the fStop value will open the aperture wider, making the depth of field shallower and blurry.  Depth of field is also dependent on the lens focal length but I believe that is not apply to Poser.