Larger the objective the better! (77 – 85 mm) = More Light!!!! Pretty self-explanatory. The smaller, compact (50-65mm) scopes are great to carry and probably provide 95% of the view you'll ever want, but they provide less light than the scopes with larger objectives. The old rule of thumb in photography states that you should shoot at a speed that matches your lens (1/500 sec. for the D70/Sigma400/1.4TC). Granted, we will be using a tripod, but we still want as much light as possible to increase our shutter speeds.
Coatings: APO, Fluorite, ED, HD APO (apochromatic) lenses are capable of converging three wavelengths of light to a single focal point. When you think Leica, you think APO Televid 77. Fluorite lenses (Zeiss), ED (extra-low dispersion glass from Nikon) and HD (high-definition glass from Swarovski) are the types of lenses one should look for in a spotting scope. They usually cost more than lenses that don't have these characteristics. Why? Their manufacturers have taken great pains to reduce the effects of Chromatic Aberration or CA. See next item, please...
Chromatic Aberration The dreaded CA is the term to describe light passing through a parabolic lens. Light is made up of different wavelengths (thus different energies) and pass through glass at different speeds, thus giving us the prism effect we cherish from Pink Floyd's Dark Side of the Moon album. Unfortunately, each wavelength converges at a different focal point, leading to what is called chromatic aberration. In a poor camera lens or spotting scope, this translates to an image with the subject surrounded by a blue-purple haze or outline.
Purple Fringing (camera micro lens CA) Purple fringing is chromatic aberration from the micro-lens of the compact digital camera. Sony (Zeiss), Nikon, and Leica put great effort into reducing CA in their cameras. Check out http://www.dpreview.com/ for extensive camera reviews and comparisons between different camera brands.
Best Scope Manufacturers (click on links for their latest product offerings)
This should by no means suggest that scopes not listed here are no good for digiscoping. These are top-end scopes and some wonderful images have been acquired from them. If you are in the market for a scope and are interested in digiscoping with it, then check out these sites for reviews and forums regarding the scopes: Andy Bright's Digiscoping Forum and Better View Desired. Bottom line? Go to an optics store or bird fair and check out the scopes for yourself! Your eyes are unique and Brand X might provide a significantly better view than Brand Y despite the ramblings of the experts.
I've used the Zeiss 85T*Fl (purchased in 2003) since I started digiscoping, and never thought I'd find a better digiscoping rig. But, since 2003 the quality of spotting scopes from all manufacturers has improved significantly. I'm now digiscoping with a Swarovski STX85 and am stunned at the increased sharpness and resolution I'm getting out of my digiscoping. No knock on the Zeiss; I just beat the hell out of the scope and it was time for a replacement.
Eyepiece - Fixed 20X or 30X Wide Angle - Zoom 20-60X It's an age-old question. Which is better? It used to be that scope manufactures made both zoom and fixed magnification eyepieces. Fixed eyepieces produced sharper images, but limited the user to a single magnification. Zoom eyepieces were more versatile, but had slightly less sharpness compared to their fixed counterparts. In the case of my old Zeiss 20-60X Zoom eyepiece, vignetting occurred between 30-60X so it wasn't really usable for digiscoping. Things are different, now!
Mostly for economic reasons scope manufactures are concentrating mainly on zoom eyepieces. Today's zoom eyepieces are incredible, though, and really raise the question whether a fixed eyepiece is even necessary. So much so that manufacturers are discontinuing fixed eyepiece production. If you have an older model scope and a fixed eyepiece then a 20X to 30X eyepiece is ideal for digiscoping.
Zeiss discontinued the 30X eyepiece, and I've been digiscoping with a 40X Wide Eyepiece for several years. Consensus is that the 40X fixed is too high a magnification for most situations, but I've gotten some wonderful images with it, albeit slightly softer than a 20X eyepiece would provide. Bottom line, best photos come from the lowest magnifications possible, so if you're starting out, set your zoom eyepiece to 20X and leave it there! I've found that cropping gives better results than increased magnification / softer images.