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Another method of connecting a D-SLR to a spotting scope has been around for many years, but film SLR cameras were used prior to digital. In this configuration, the eyepiece is removed from the spotting scope and the lens is removed from the D-SLR. Using a T-ring that's specific to the camera body, the scope manufacturer's photo adapter is connected to the D-SLR. The opposite end of the photo adapter is connected to the spotting scope.
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An advantage to this setup is its simplicity and universality – just about any SLR/D-SLR can be used. Though the focal length rendered is 800mm, it's optically slow at f/10 (from 800mm / 80mm). However, in good light this method will deliver very nice results. A notable disadvantage is that it's much more cumbersome to convert the scope from observing to photographing (and back) while in the field.
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This method is available in all major spotting scope models; Leica, Kowa, Zeiss, Nikon (only Nikon's D-SLRs will work), Swarovski and more. Though I have a photo adapter for my Swarovski scope, I've only used it with my Yashica film SLR. When I eventually buy a D-SLR, I'll probably revisit this method but I want to make sure the camera I choose will also a-focally couple to the eyepiece, like the Pentax K100D does, for slightly longer focal length.
Here's why:
Because the eyepiece is attached, the Swarovski 80mm scope at 20x starts with a focal length of 1,000mm (from 20x times 50). The sensor size on the K100D is 23.5mm, so there is a crop factor of 1.48 (35mm/23.5mm). This factor is multiplied by the 40mm lens (40mm x 1.48) for 59.2. Next, we convert back to 35mm equivalency (59.2 / 50) and get 1.184. This is multiplied by 1,000mm for a final effective focal length of 1,184mm. This is pretty close to the digiscoping magnification when I use my Nikon Coolpix 995.
All images © 2007 Mike McDowell