Relative brightness is a measure of how effectively binoculars or optical devices perform in low-light conditions. It is calculated by dividing the diameter of the objective lens by the magnification and then squaring the result. For example, a 10x40 pair of binoculars has a relative brightness of 16 (40 ÷ 10 = 4; 4 x 4 = 16).
A higher relative brightness value indicates better performance in dim lighting, making it easier to see details at dawn, dusk, or in shaded environments. Photographers and wildlife enthusiasts often consider relative brightness when choosing binoculars for spotting subjects or planning night or low-light photography sessions.