Jack Congson , Jun 29, 1999; 06:19 p.m.
As far as I know there are basically 2 kinds of prisms for the RB, the old style and the new style - pd prism. The big difference is that the new style allows the installation of the eyepiece magnifier. The old one has to be upgraded ( about $85 ) to accommodate the magnifier. In my opinion, for critical focus, especially with wide angle, the magnifier is indispensable.
I used to have an old style prism on my RB but after getting the scare of my life when my heavy prism decided to come loose of the body and fall with a thud on the ground I decided the Chimney viewfinder made of very light plastic with a built-in flip-up magnifier for critical focus was going to permanently sit on top of my RB Pro-S body.
The screen is now much brighter and the large eyepiece is great for looking at the focusing screen. Now it is even easier to handhold.
For the big RB, I can't really see the practical need for a prism. It is not a camera normally used to follow action therefore the reversed image of the chimney finder is just a matter of getting used to.
With my lighter 330S that I use with a 55mm for candids, I prefer to have a prism which in this case almost weighs nothing to fillow the action.
With the big RB, no way I am going back to use a heavy prism. I really feel that heavy piece of glass just stymies my shooting style especially on location.