One question I get often is, “Can I digiscope with a zoom eyepiece?”
The answer is, yes, though it is always something of a compromise to digiscope when your spotting scope is fitted with a zoom eyepiece. You will get some vignetting (darkening at the edge of the field) or, worst case scenario, portholing (a round frame pic, instead of a rectangular frame). These effects be edited by cropping with your photo program, however.
Of course, not all spotting scope zoom eyepieces are created equal. High performance spotting scope zooms, such as those offered by Swarovski are not only optically excellent, they also have larger lens elements, making them a better match for the lenses on today’s digital point and shoot cameras. Inexpensive spotting scope zoom eyepieces are optically challenged and suffer from tiny lens elements, making them the worst possible eyepieces for digiscoping.
The best eyepieces to use for digiscoping? Most digiscoping nuts like me prefer fixed power, wide-angle eyepieces. I use a TSE-21WD 20x wide angle on my Kowa 663 spotting scope for most of my shots (though I also use the excellent TSE-Z9B zoom on occasion). Eyepieces like this have the widest lens elements you can get in a spotting scope eyepiece and also offer maximum eye relief. This means less vignetting and more flexibility with camera placement behind the eyepiece.