I Can See For Miles, Pt. II: A Brief History of Prism Binoculars

Welcome to part two of the Sandy Mountain Historical & Technological Society’s brief history of binoculars. In the gripping cliffhanger finale of part one, we introduced the prism binocular design, which was developed as a method of righting the inverted images presented by the Keplerian optical design. Today, our tale concludes with a closer look at the technology that changed the long-distance viewing game forever.

Go Go Porro Bro Bro

Porro prism binoculars are the kind of field glasses you most likely picture your grandfather using. (Go ahead and conjure up a quick mental image of that. We’ll wait.) Named for the Italian inventor who developed this particular lens configuration in 1854, Ignazio “Big Iggy” Porro, this type of binoculars uses Porro prisms (also developed by Big Iggy) arranged in a Z-pattern to flip the image the right way ‘round. By necessity, Porro prism binoculars are wide, with widely-spaced objective lenses (the ones you point toward what you’re looking at) that are offset from the ocular lenses (the ones you look through, also called eyepiece lenses).

Likely the greatest photo of Porro prism binoculars ever taken.

Likely the greatest photo of Porro prism binoculars ever taken.

One significant benefit of the Porro prism design is that its folding optical path (created by the Z-configuration of lenses and prisms) allows the binoculars to have a focal length that is longer than the physical length, sometimes significantly longer. The wider space of the objective lenses also gives the presented image a better sense of depth.

Cat On A Hot Tin Roof Prism

Developed in the 1870s by noted great-French-name-haver Achille Victor Emile “Big Vic” Daubresse, roof prism binoculars use either the Abbe-Koenig prisms (invented by German engineers Ernst Karl “Big Ern” Abbe and Albert “Big Al” Koenig, and later patented, Thomas Edison-style, by Carl Zeiss in 1905) or Schmidt-Pechan prisms, a.k.a. Pechan prism pairs to invert the image to its proper orientation. In this configuration, the objective lenses are aligned with the eyepiece lenses.

Military-grade roof prism field glasses.

Military-grade roof prism field glasses.

The roof prism design results in field glasses that are narrower and more compact than their Porro prism cousins. And, whereas Porro prism binoculars occasionally need their prisms realigned, the fixed alignment of roof prism models eliminates the need for realignment, except in extreme circumstances. However, due to the silvered surfaces of their lenses and prisms, roof prism binoculars present images that are generally 12-15 percent darker than equivalent Porro prism binocs. Additionally, due to the tighter tolerances required for proper alignment of the optical elements, roof prism binoculars tend to be more expensive than the Porro prism variety.

Top photo credit: The U.S. National Archives via Foter.com / No known copyright restrictions
Bottom photo credit: Caro’s Lines via Foter.com / CC BY-NC-SA