What is a roof prism?
A18. A roof prism is a smaller prism that is shaped like a house with a roof. You can always recognize a roof prism by its shape. Unlike porro prisms, the objective lenses are approximately in line with the eye pieces. The main advantage of a roof prism is its size and shape. It is smaller and more compact than a porro prism and can be installed in a sleeker housing than a porro prism. This makes a roof prism binocular a nicer binocular to hold and handle for most people. Conversely, a roof prism is much more expensive to make to the same optical standards as a good porro prism because they employ silver coated mirrors that reduce light transmission by as much as 15%. Porro prisms do not use mirrors and do not have the light loss factor to contend with that roof prisms have to as a result. Roof prisms also require tighter tolerances for alignment of their optical elements (collimation), further adding to their expense because the design requires them to use fixed elements that need to
