You can have light boosting binoculars with 1x zoom. Similarly, looking through the viewfinder of an SLR camera will show the light- gathering capability of the lens. The very best light gathering camera lenses (and binoculars) only collect 2 to 4 times more light for a given focal length compared to the human eye. Since our vision is so nonlinear in sensitivity, the 2-4x boost doesn't seem like much. The brightness theorem only says you can't "perceive an object to be brighter than it is at its surface" (my interpretation)