Lowest useful magnification: The lowest power usable from a dark sky observing site is about 3x to 4x per inch of aperture - for viewing open clusters and rich field scanning of the Milky Way star clouds. An eyepiece with still lower power has an exit pupil (the beam of light coming out of the eyepiece) larger than the pupil of your eye. A portion of the exit pupil falls on your iris, instead of entering your eye, effectively reducing the aperture (and brightness) of your scope.