What is relative brightness?
The relative brightness index is used to compare how well binoculars with different size exit pupils will perform under dark conditions. This index reminds us that as the size of the exit pupil increases, its area and ability to transmit light grow geometrically. To find the relative brightness, square the exit pupil. A binocular with an exit pupil of 5mm has a relative brightness of 25 (5×5=25). Because relative brightness does not consider factors such as optical quality or coatings, it should be used only as a rough guide.
