I realize this is after the fact. IMO the 45 degree PM-5 prism is the most practical. If someone is looking for a 90 degree and you can find an RM-2 it does enlarge and has a +/- 5 degree diopter eyepiece. It comes in handy esp. with macro or long lenses with a distant subject. Schneider makes (or made -- I've had mine for years) a 6x6cm magnifying loupe that can be used for viewing negatives or will fit the Hasselblad 500 series and work as a viewfinder similar to what was shown.