The things you need to know are:
Tthat larger is weaker, so that 2X 'Sherlock Holmes' magnifier is not very useful, except as a reading glass.
Anything over 10X becomes hard to focus.
A light incorporated into the glass makes it much more efficient.
Some magnifiers use two lenses (a 2X and a 10X for example) which can be used separately or together .
A magnifier which can be closed when not in use will stay unscratched.
For well under $20 you can get magnifiers that fit all of these criteria from Lighthouse in Montreal or Unitrade in Toronto. Both are online.
