The object is to get as much light as possible entering the binoculars through the objective lenses eventually entering the viewer’s eyes. This is achieved by the optical quality of the glass used for the lenses and prisms, as well as the optical coatings. Lenses and prisms should be made from BaK-4 optical glass (better than BK-7), which has even become common among cheaper binoculars nowadays.
Optical coatings can be either “coated”, “fully coated”, multi-coated or “fully multi-coated”. The best is “fully multi-coated”, but these binoculars are more expensive. On top of this roof prism binoculars should also have “phase-corrected” coatings. The impact of lenses, prisms and lens coatings is discussed in Understanding binoculars 3.