A prism is a transparent glass optical element with flat, polished surfaces positioned at that refract light. The purpose of prisms is to give you an upright image; they are to correct the inverted and reversed images you would see in their absence. There are two prism designs: Roof and Porro. 

Both Porro and roof prism designs are commonly used in the creation of binoculars, spotting scopes, and monoculars. However these two designs are quite different, with each providing their own benefits and costs.

 

 

The Porro prism, named for its inventor Ignazio Porro, is a reflective optical prism in the form of a right-angle triangle with two sides of equal length. In binoculars, monoculars, or spotting scopes, it is used in pairs to lengthen the effective focal length of the optical path within a space that is shorter than it would otherwise accommodate. As each turn of the optical path through the prism is 90° the Porro prism does not risk dispersion of the light rays as they pass through it, yielding an image presented to the user that is very natural, bright, and true to the natural colors of the subject. Porro prism optical instruments are easily recognized by their having eyepiece lenses that are not in a straight line with their objective lenses, consequently, they are often physically larger than roof prism models of equal magnification and objective diameters. The classic “broad shouldered” shape of a traditional binocular is a readily identifiable clue to that model being a Porro prism optical system.

The roof prism, also called a Dach prism, is a reflective optical prism containing a section where two faces meet at a 90° angle. These two 90° faces resemble the roof of a building, giving this prism type its name. However unlike the Porro prism, which is generally used in identical pairs, the roof prism is used in combination with an auxiliary prism both to lengthen the effective focal length of the optical path within a space that is shorter than it would otherwise accommodate, and to do so in a manner that creates a straight line between the eyepiece and objective lenses of the optic for the result of a sleek and trim exterior shape. The cost of this is that most of the angles the light path must follow through the roof and auxiliary prisms are either very sharp or quite wide, which increases the risk of the light rays dispersing in the process. To overcome this, special coatings such as Phase Coating and Dielectric Coating are used to keep the light rays transiting in proper alignment to one another and thus show the user of the optic a crisp and sharp image in the eyepiece.