Vortex Diamondback Binoculars 9x36 Phase corrected prisms (increasing light transmission by 7%) deliver sharp, greatly contrasted images. BaK-4 prisms provide sharp images from one edge of the broad viewing field to the other. Fully multi-coated optics increase light transmission for higher image quality in all lighting conditions - enhanced for 2009. Complete waterproof protection keeps dust and water out of the optical system with tight O-ring seals. Complete fogproofing of internal lenses with argon-purged optics presents clear views in spite of sudden temperature changes.