I use what my camera brand offers. Angle viewfinder doesn't really give you a lot of magnification IIRC. I use a angled viewfinder because I want the camera to stay lower. Can't really think of how it can help in magnifying a subject for crops. Can you elaborate how much of a crop you going to perform or what you trying to achieve?
My suggestion:
1) Change the focusing screen to a split-image screen. Phone OP and check stocks. They may also be able to help you install the screen.
2) Check out angle finders. They offer variable magnifications of between 1.25x/1.5x to 2.5x/2.8x. The Seagull is really clear, but tends to induce eye fatigue. The Canon Angle finder C is crystal clear and doesn't make the eye so tired. It's about 3x the price of the Seagull though.
3) Try a viewfinder magnifier. They usually magnify only the central portion at 2.3x to 2.8x. These can be gotten on ebay. I have a spare if you're interersted.
I use all three of the above as appropriate.
Generally, any magnification of 2x and above is sufficient for accurate focusing. Couple that with a split-image screen and manual focusing gets much easier, except in dark situations.
Thanks for your brief review. I think I will go for Canon Angle Finder C. Do you spare to sell me?
3) Try a viewfinder magnifier. They usually magnify only the central portion at 2.3x to 2.8x. These can be gotten on ebay. I have a spare if you're interersted.
I use all three of the above as appropriate.
Generally, any magnification of 2x and above is sufficient for accurate focusing. Couple that with a split-image screen and manual focusing gets much easier, except in dark situations.
An angle finder is probably not the best solution for your problem. I'm not sure why you arrived at the conclusion to get one. Your best bet is still the split-image screen, as these have been in use by SLR users from the manual focus days.
Sorry to ask you a simple question. What does the split image sceen do?