What is the difference between roof prism and Porro prism Spotting Scopes?

While descriptions of binoculars usually include a reference to the type of prism system used in their respective optical designs (Porro or roof), spotting scopes generally do not have these prisms identified clearly. However just as in binoculars, spotting scope optical systems also rely upon well-designed prism systems to enable them to provide the levels of high magnification and image quality that they do without being many feet in length.

As a straight body spotting scope is essentially half of a binocular that provides a much higher level of magnification than most binoculars do, it stands to reason that they would often use the same types of prisms as are used in binoculars. For example, the Celestron TrailSeeker and Ultima straight body spotting scopes all use two Porro prisms – the same style as is used in the Ultima or SkyMaster binoculars, only larger, - as essential optical elements in their design.

However when angled spotting scopes are created, the type of prisms used changes to meet the additional optical requirements of these designs. In smaller spotting scopes, such as the Celestron Landscout and Hummingbird models, for example, roof prisms are used to allow these scopes to be as small as they need to be for maximum portability and convenience.

For larger spotting scopes, such as the Regal ED Angled, TrailSeeker Angled, and Ultima Angled models, large Porro prisms are essential to the optical design, however an additional prism is also added in order to bend the optical path the necessary 45° to enable it to fit the physical design of the spotting scope. This is accomplished by a prism the shape of a pentagon cut in half (called a half pentagonal) being added to a pair of Porro prisms to achieve the desired optical design.

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