What is Twilight Factor and Relative Brightness and how do I Calculate it?

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 a 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 8x42 binocular = 18.3, while the twilight factor of a 10x42 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 8x42 binocular with an exit pupil of 5.25 has a relative brightness of 27.6, while a 10x32 binocular with an exit pupil of 3.2 has a relative brightness of 10.2

Other knowledgebase article you might be interested in: What determines the brightness of the image in my binoculars?