A Porro prism is an optical glass prism designed to reflect a beam of light through a total of 180 degrees by means of two internal total internal reflection surfaces set at 90 degrees to each other, effectively folding the light path back on itself and laterally displacing it without inverting the image. Named after the Italian optician Ignazio Porro who patented the design in 1854, the Porro prism and the binocular designs based upon it have become one of the most widely recognised and enduring optical instrument configurations in the history of optics.
In the conventional Porro prism binocular design, two Porro prisms are used in each optical tube in a paired arrangement with their reflection planes oriented at right angles to each other, collectively folding the light path through 180 degrees while simultaneously correcting both the lateral and vertical inversions that the objective lens introduces. The folded light path of the Porro prism arrangement also allows the objective lenses - which must be large enough to gather sufficient light for a bright, detailed image - to be spaced wider apart than the eyepieces, giving Porro prism binoculars their characteristic stepped or offset body shape in which the objective lens barrels are set wider than and offset from the eyepiece barrels. This wider objective spacing actually provides a slight enhancement of the stereoscopic depth perception experienced through the binoculars, as the greater separation between the two objective lenses exaggerates the parallax difference between the left and right eye views beyond what normal human interocular separation would produce.
The Porro prism binocular design offers several optical and practical advantages. The relatively simple geometry of the Porro prism allows it to be manufactured to high optical quality at modest cost, and the design's inherent optical efficiency - total internal reflection within the glass prism being a virtually lossless reflection process - means that Porro prism binoculars can deliver bright, high contrast images with excellent light transmission at competitive price points. The wide objective spacing of the Porro design also allows the use of larger objective lenses without making the instrument excessively wide or heavy, contributing to good low light performance.
The principal alternative to the Porro prism design in modern binoculars is the roof prism system, used in the slimmer, more compact straight-tube binocular designs that have become increasingly popular for travel, birdwatching, and general outdoor use. In a roof prism binocular, the prism arrangement folds the light path in a way that keeps the objective and eyepiece in direct axial alignment, allowing the binocular to be made in a straight, compact, slim-profile body in which the objective and eyepiece of each barrel are in line with each other. While roof prism binoculars offer the significant practical advantages of a more compact and pocketable form factor and a more robust, sealed body design, achieving equivalent optical quality to a Porro prism design requires more complex and precisely manufactured prisms - particularly the phase correction coating required on roof prism surfaces to maintain image contrast - making high quality roof prism binoculars typically more expensive than equivalent Porro prism designs.
In the context of photography, binoculars of both Porro and roof prism types are widely used by photographers as a complement to their camera equipment, particularly in wildlife, nature, bird, and sports photography where the ability to scan and locate subjects at distance before raising the camera is an invaluable aid to finding, identifying, and anticipating photographic opportunities. The choice between Porro and roof prism designs typically involves a trade-off between optical quality and value on one hand - where Porro prisms generally offer an advantage - and compactness, durability, and portability on the other, where the straight tube roof prism design excels.