LAST EDITED ON Dec-16-09 AT 01:16PM (MST)[p]What I have found, and this is with any scope, is that optical performance starts to fall of noticeably when the exit pupil falls below 2.0 mm. More expensive scopes show less of this, but it is nonetheless there. I have looked at (but not owned or used very extensively) the 15-30x50 Leupold. Those I looked at were by far the best at 20-25x as the upper limit, and then fell off. However, I like the scope as a small portable unit and as a practical matter 20-25x will do a lot.
The thing really is, to get real good performance at levels of magnification above 30x, you really need to get into the bigger 60-80 mm scopes. While optical quality can improve, portability goes to pot. Another thing in my experience (as has been mentioned in another post) is that heat distortions (mirage) can destroy any usefulness of optical instruments of any sort much over 10x, if the conditions are right. Mirage is one reason why I like a 7x for my main binocular, as I hunt in the high sage-juniper desert country and mirage can be a bugger at times.