Whenever you increase the magnification of an optic, the imperfections of the image get worse even faster. Lots of things influence this, but it boils down to how well it works.
Nowadays, it is possible to get very good performance from some of the cheaper optics. The best advice I can give you is buy from somebody that will take it back for a full refund if you aren't happy.
What are we talking about here, spotting scopes ? E
