PLEASE -- PLEASE go for the Optivisor by Donegan Optical. We have used these in our electronics lab for years and I've noted that many Doctors and Dentists also use them. One big advantage of these is that they have interchangeable lens plates for different levels of magnification. They are also made of real optical glass and not prone to scratching as are the plastic ones.
P.S. I have no interest in the firm except as a customer and they are made in the U.S.A.