What is “light gathering power”?
Objective lens size is the factor that determines how much light can potentially be collected by the binocular, and thus how deep you can see in the sky itself. Note that how much light actually gets to the observers eye is dependent on several factors, such as prism, size, baffling, and coating quality. In general, though, given similar coatings, you can use aperture to compare how much light will be gathered by two different binoculars.
