Ellen,

About Polaroid backs and Medium Format...
I almost purchased one of those backs for my RB-67 but was able to borrow one from a friend first and I'm glad I did. After about two packs of Polaroids and three rolls of film I realized I didn't need it anymore. I was shooting with one monolight and metering in my studio and everything just sort of gelled. I pretty much know what I'm going to get now and felt like that back was just a waste of money.

Shooting with the 4x5 is the exact opposite.