Relative brightness is a theoretical estimation of how bright the image should be when viewed through binoculars or telescope. It is calculated by simply squaring the exit pupil value. The exit pupil is the size of the beam of light that comes out into your eye. Sometimes, this value is also reported with the binocular or spotting scope specifications, but it is also easily calculated by dividing the size of the objective lens by the magnification.
For example (see Table 1), an 8 x 50 (8 power magnification and 50 mm objective lens) binocular has an exit pupil value of 6.25 (50/8). Compare that to 10 X 50 pair of binoculars, where 50/10= 5.0 exit pupil. Then square each exit pupil value to get relative brightness values of 37.5 for the 8 X 50 and 25.0 for the 10 X 50 binoculars. The 10 X 50s have only 67% of the relative brightness of the 8 X 50 binoculars.
See also:- What do the numbers on my scope mean?- What is magnification?- What is the field of view- What is an objective lens?- What is eye relief?- What is the exit pupil?- Twilight Factor- Relative Brightness