What are the benefits of using old digital back (by old I mean devices that has 6x4.5 sensor size and 20-50 mpix resolution) on a medium format camera like Mamiya RZ67, instead of using modern full-frame camera like Canon EOS R5?
Digital back might be a little bit cheaper than a new FF body, but we have to remember you have to have lens, body, viewfinder etc on one side and again lens on the other side.
Am I missing something?
If you start from scratch and need to obtain the equipment, there is probably no advantage, but rather a lot of disadvantages. Used, but high quality MF film cameras are not necessarily cheap, so if you get a used MF camera and digital back, you are not likely to pay very much less than for new 35mm format equipemnt.
If you on the other hand already have a good, suitable MF camera and perhaps a selection of lenses, it is likely much cheaper to buy an adapter and an older digital back than to buy new 35mm format equipment with similar image quality. I have a Mamiya RZ67 with a lot of very good glass and I could get an adapter and a very descent older digital back (e.g. Aptus or Phase One) in the 30-40 MP range for that camera for much less than what only a Canon EOS R5 body would cost. If I for example still want to use the MF camera for film, don't bother having much heavier equipment to tug around and don't need the probably much better low-light performance of a new 35mm format camera compared to the older MF digital backs, it would surely be something to consider.
