I Can See for Miles: A Brief History of Field Glasses

Binoculars. Binocs. Field glasses. The ol’ spysie eyesies. Whatever you call them, these popular, (mostly) handheld, binocular telescopes have been around for centuries, essentially since the invention of the boring ol’ monocular telescope (commonly known as the telescope). Which makes sense, since, really, just put two regular telescopes side by side and you’ve pretty much got binoculars.


“Whaddaya see out there, Mildred?”

Galileo Figaro!

The earliest field glasses utilized “Galilean optics”—a convex objective lens (the one you point at what you’re looking at) paired with a concave eyepiece lens (the one you look through, also called the ocular lens). While Galilean optics do present magnified images oriented in the correct direction, unlike the camera obscura and other optical devices that present upside-down images, they provide only a very narrow field of view, and cannot provide particularly high magnification.

The same basic design is still used in modern opera glasses and el cheapo kiddie binoculars, as well as surgical and jewelers’ loupes that don’t require high magnification. The technology remains popular for such applications because they can be very short and produce clear, upright images through a simple design.

Put Some Kepler in Your Step

A significant improvement was made through the use of Keplerian optics, a design used in the first refracting ocular telescopes. Because the image formed by the objective lens is viewed through a positive eyepiece lens, Keplerian optics provide higher magnification and a clearer image. However, this configuration does produce an inverted image, and requires an additional lens or lenses to present a correctly-oriented image.

Erecting Lenses (Oh, Hush…)

Aprismatic field glasses using Keplerian optics have either one or two additional relay lenses betwixt the objective and ocular lenses. By necessity, the design of these binoculars made them too long for practical use. They became obsolete with the invention of prism binoculars (see below) in the late 19th century CE and quickly fell out of use.

Prism Isn’t Just A Disappointing Katy Perry Album

The new-and-improved binoc design, known as prism binoculars, utilize prisms to invert the image so that it is presented right side up. There are two common prism binocular variations: porro prism and roof-prism. Tune in next week to learn about these game-changing field glasses.

Photo credit: MLazarevski via Foter.com / CC BY-ND