Marathon Compact Roof Prism Binocular 8 x 21. The BK-7 prism is the most widely used prism amongst the binoculars. These binoculars are light-weight and can be folded, very convenient and easy to carry. Perfect for bird watch, travelling, sightseeing, hunting, observation of sporting activities and concerts, etc. Comes with neck straps, fold-down eye cups, lens cloth, carrying pouch, and full instructions.