What determines the brightness of the image in my binoculars?
February 20, 2005
Brightness is important to consider when comparing different pairs of binoculars, as it impacts your overall ability to see things (sky objects, animals, and landscapes) in low light. The quality of the image contrast and color also depend on brightness.
The physical factors determining a given pair’s image brightness are the objective lens diameter, the light transmission of the optics and coatings used on the lens surfaces and prisms throughout the binocular, and the magnification. Larger objective lenses will gather more light, and modern coatings allow more light to pass through the optics. When comparing two pairs of the same objective size, a lower magnification will have greater brightness: a 7x50 binocular will have a brighter image than a 12x50.
If you want to compare numbers before looking through actual binoculars, there are several calculations to determine brightness or the effectiveness of binoculars in low light. The first is twilight factor, found by multiplying objective size by magnification and taking the square root of the resulting product. For example, the twilight factor for a pair of 7x50s is 19. The larger the twilight factor, the easier it is to see detail in low light.
A second measure is the relative brightness index, or RBI, which is the square of the exit pupil (objective size divided by the magnification). Those same 7x50s would have an exit pupil of 7 and a RBI of 51.
Finally, RLE is used when comparing coated to uncoated (vintage) binoculars. To calculate RLE, just increase the value of the RBI by 50%, so the RLE for coated 7x50s is 77. Binoculars with higher RBI and RLE numbers deliver brighter images.