Telephotography with Spotting Scopes
Although the primary purpose in purchasing a spotting scope is for visual observation, the capability of its use as a telephoto lens is an important feature offered with most models. The optical design and configuration of the scope and the available accessories will determine its performance and effectiveness for this purpose. Some important differences should be noted between a spotting scope used as a telephoto lens and a typical camera telephoto lens. As opposed to a camera telephoto lens, spotting scopes will usually operate at much higher magnifications and at a fixed aperture or f/number, resulting in a greater need for stability and the use of higher speed film and/or slower shutter speeds for correct exposure. Also, there is usually a more limited depth of field, a narrower field of view and the preview image in the camera viewfinder will not be very bright for obtaining proper focus. Despite some of these limitations, excellent photographic results are possible at fairly high magnifications. The telephoto range of spotting scopes will vary from about 500mm (10X) to 4000mm (80X). The photographic speed will usually decrease (higher f/numbers) as the magnification increases, with a general range of from f/7 to f/64.
Mounting a camera to a spotting scope usually requires two separate mechanical attachments; a photo adapter which is attached to the back of the scope and a specific T-Adapter (T-ring) to fit on to your camera body. These two parts are connected together and a slip ring or swivel device will allow for the desired camera orientation. Telephotography with a spotting scope is practical only with 35mm SLR cameras which use interchangeable lenses.
Thanks to our friends at Eagle Optics for providing this information!
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