A binocular’s brightness can be affected by several things. Contrary to what many binocular users believe, big objective lenses are not the only factor to consider. The quality of glass used in binocular prism and lens construction, along with the quality of the lens coatings, contribute more to brightness than do big objectives.
Lens coatings are thin layers of chemicals applied to the surface of a binocular’s lenses to reduce light reflection and improve light transmission. Brightness of the image is affected by the number and coverage of the coatings, on all air-to-glass lenses, inside and out.
Phase-corrected binoculars have prisms that are treated with a set of coatings that keep light in correct color phases. These coatings (only applied to roof prism binoculars) enhance resolution, contrast, and color fidelity.
Ideally, you should purchase a binocular that is fully multi-coated, which means both sides of every lens are coated with at least several layers of anti-reflective chemical. High quality lens coatings will appear as fairly light, subtle shades of blue, green, or violet.
Note: If all other things are equal in two binoculars, the one with the larger objective lens will yield a brighter image, but at the cost of greater size and weight. Because of exceptional glass and coatings, higher quality binoculars with a smaller lenses may very well appear brighter than less expensive binoculars with larger objective lenses.