What objective size?
The amount of light that passes through your binoculars and reaches your eyes is dependent on two things: (a) the size of the large lens at the end of the binoculars (the objective) and (b) the coatings on the lenses of the binoculars.
- The more magnification, the larger the objective size (measured in millimeters) you need for adequate brightness. For example, a pair of 8 x 32 binoculars and a pair of 10 x 42 binoculars will have about the same brightness. This is called “exit pupil” and calculated by dividing the size of the objective lens by the magnification. Thus, 8×20 binoculars have an exit pupil of 2.5, 8×32 binocs have an exit pupil of 4, 10×20 binoculars have an exit pupil of 2, and 10x 42 binoculars have an exit pupil of 4.2. The larger the exit pupil index, the better!
- As light passes through the lenses of the binoculars, a small amount ricochets off the glass. More expensive binoculars have multi-coated lenses that minimize this effect helping the image to be brighter. The difference in a $150 pair of binoculars and a $3,000 pair of binoculars is due to the quality of these lens coatings as well as the durability of the housing.