Twilight Factor is a number used to compare the effectiveness of binoculars or spotting scopes used in low light. The larger the twilight factor, the more detail you can see in low light. Magnification plays a critical role in the twilight factor calculation as higher power will provide you with much greater detail and image identification. Note: This does not take into account optical quality or coatings.
Twilight Factor Equation is found by multiplying the size of the objective lens (in mm) by the magnification and then finding the square root of that result. For example, the twilight factor of an 8×42 binocular = 18.3, while the twilight factor of a 10×42 binocular = 20.5
Relative Brightness is a number used to compare the brightness of binoculars or spotting scopes of similar magnification. The larger the relative brightness number, the brighter the image.
Relative Brightness Equation – Relative Brightness = Exit Pupil² (or Exit Pupil size in mm, multiplied by itself). For example, an 8×42 binocular with an exit pupil of 5.25 has a relative brightness of 27.6, while a 10×32 binocular with an exit pupil of 3.2 has a relative brightness of 10.2