The difference in the two types of binoculars refers to the arrangement of the prisms inside the binoculars, which affect the path of light. Binoculars that use roof prisms maintain what appears to be a straight path of light from the objective lenses back to the eyepiece lens. Binoculars with Porro prisms have objective lenses offset from the eyepiece lenses. In other words, the light path is folded inside binoculars with Porro prisms.