You know when people say "the power of my glasses is 150"???
Actually, that is a very local term (Malaysia/Singapore).
The correct terms is that "the correction for my eyeglasses/lenses is -1.50 diopter"
So, it is the amount of optical refraction, either convergent or divergent, that a corrective lens has (something like that anyway).
OK, I have been using glasses for over 33 years. And I have been using film and dig SLRs (Pentax, Minolta, Nikon, Canon etc) for the past 30 years. And I have never had a single problem with the viewfinder.