It focuses the image to make it visible to your eye. The larger the objective, the more light the binocular will gather. Prisms - Binoculars use convex lenses for both the objective lens and ocular lens, which results in an image that appears upside down and backwards. But Pentax puts a seven-layer coating on all optical surfaces. This means more available light is transmitted to your eye.
Glare is reduced and image contrast is enhanced.