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Different Types of Binoculars

After the telescope was invented, it didn’t need a huge leap of imagination for man to mount them side by side, one for each eye, thus making the first binoculars. But as times have changed, so have the ways of making binoculars.

Galilean binoculars are the aforementioned binoculars that consist of two telescopes side by side. You might be surprised to know that their name does not come from the fact that Galileo invented them, but that they use his system of optics; a convex objective lens and a concave eye lens. In other words, they are refractor telescopes. Galilean binoculars suffer because they have a narrow field of view and cannot change their magnification much (if indeed they can at all), but the benefit is that they are the cheapest form of binoculars, and the simplicity of the technology means that they are often used for other applications, such as opera or theater glasses.

Prism binoculars are the next stage of evolution in binocular technology. The name is rather deceptive – what prism binoculars do differently from Galilean binoculars is that they use a convex eye lens instead of a concave eye lens. This arrangement causes the rays from the eyepiece to converge, instead of diverging. The benefits of this are twofold – a much wider field of vision and improved eye relief. However, the image produced is inverted, and must be turned right side up by other means.

Porro prism binoculars are one of the more common binocular types nowadays. As expected, porro prism binoculars use porro prisms, which are essentially halves of a glass block cut at a 45 degree angle such that light which enters from the top half of the prism’s hypotenuse reflects twice off the internal surface of the prism and exits from the bottom half of the hypotenuse, in exactly the opposite direction by which it entered. Porro prism binoculars typically use to of these prisms per optical chamber in a binocular. Porro prism binoculars tend to have objective lenses spaced very wide apart, which enhances depth perception, and they tend to be shorter, because the arrangement of the prisms causes the light path to be folded, which maintains a long focal length while not unduly affecting the size of the binoculars.

Roof prism binoculars are the newest kind of binoculars. Much like porro prism binoculars, they bounce the light within them in order to fold the optical path and reduce the size of the binocular. Unlike porro prism binoculars, they are much more compact, and the eyepieces are roughly in line with the objective lenses.

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