Similar Terms: BAK-4, Porro Prism, Prism, BAK4
BAK-4 prisms are considered the best type of Porro prism as they are made of superior optical glass to produce clearer images. BK-7 prisms are also used, usually in lower priced binoculars. These are satisfactory, but inferior to the BAK-4 prisms.
Binoculars with BK-7 prisms can be judged by a squared-off side to the general roundness of the image. BAK-4 prisms show a truer round, which translates to better light transmission and edge-to-edge sharpness.
Porro prism binoculars can usually be identified by their offset tubes; the objective lens is not in Line with the ocular lens. The other commonly used Prisms in binoculars; roof prisms, result in straight tubes (the front/ objective lens is in Line with the rear/ocular lens) and consequently these binoculars can be more compact, an important consideration for some sportsman.