A rave Zeiss review!:
A couple of years ago I sold my Swarovski 65mm 20-60X scope and bought the straight 65mm Diascope with the 15-45X eyepiece. I'm glad I did. The 65 Zeiss is relatively light and easy to carry around on its Bogen tripod, and I really like the dual focus system, compared to the barrel-type focusing of the Swaro. The image quality is superb, as you would expect, but the 15-45X eyepiece is the best part. The brightness, field of view and depth of field of the 65 Zeiss scope at 15X makes it easy to find things and to follow birds in flight. I will always remember a wonderful twilight hour I spent watching a pair of short-eared owls hunting over a marsh. I was able to see and follow the owls until well after sundown on a cloudy day, after other birders had packed up and left.
On the other end of the zoom range, I have found that 45X is plenty for shorebirds and such, and provides usable views over fields, water or snow when images at 60X turn into a dark, fluttery mess due to mirage and the tiny exit pupil at 60X. I gave up the 60X to gain the 15X, and the tradeoff has worked for me.