Not all the light arriving at the objective lenses at the front actually passes through – some is reflected. This happens with the light finally arriving at the viewer’s eyes at the other end of the binoculars as well – not all the light travels through the final two lenses either. On top of this the other 10+ glass surfaces inside the instrument contributes to scattering of light as well. Anti-reflective optical coatings reduce internal light loss and glare and ensure even light transmission, resulting in greater image sharpness and contrast.
These coating differ substantially in quality: “Coated” simply means a single layer anti-reflection coating on the two objective lenses; “Fully coated” means that all air-to-glass surfaces are coated, whereas the ideal coating, “fully multi-coated” means all air-to-glass surfaces have multiple layers of coats.
In the case of roof prism binoculars the lenses also have to be “phase-corrected” (PC) to ensure that the loss of image brightness and contrast is corrected.