Brightness: This is an especially important consideration for deer hunters, who are apt to do a lot of glassing in the dim light surrounding dawn and dusk. Though the quality of the lens coatings plays a role, brightness is largely a function of the exit pupil, which is determined by the size of the objective lenses divided by the power. For example, 8x32s and 10x40s both have an exit pupil of 4mm, and therefore have the same relative brightness. The 8x32s, though, will be slimmer and lighter. Again, this is a good reason not to overdo magnification. To match the brightness of 8x42s, for example, a 12x model would have to have 63mm objective lenses--which would make for a binocular few deer hunters would want to lug around.