I shoot small format, medium format, and large format.
On the other hand, if I were starting from scratch, I would gladly invest in a good 6x4.5 system because it is an excellent compromise between a 35mm small format system and a larger medium format system.
The main problem I'd find carrying MF and 35mm is "film commonality". Two MF cameras of different formats can use the same pool of film.
Fuji did have an interesting complementary approach in their Medium format. 6x9 as a large frame with the all manual Texas Leicas GW690 and then a more convenient (first metered, later AF P&S) 645 "system". There is the possibility of interesting combinations there, choosing wide/tele in each of them.
I only have a GW690 and do feel there is a gap that a 645 would fulfill nicely, maybe even disposing of 35mm.
Now I am at 36 Exp 135 and 8 Exp 6x9 MF. 15/16 Exp 645 is actually a good point.
I've got the idea of sometime doing a SE Asia trip and the quantity of rolls to bring does give me some thought, considering carry limits.
On the topic of 220. Now it's all but gone and expensive, 7 yrs ago it was at the current price of 120 approximately. That converts a 645 into a 30 exp camera. The 6x9 would have a respectable 16 exposures.
OTOH, 645 is not 645 is not 645. I've seen Mamiya 645 and they aren't really small. Granted the GW690 is big, but not that terrible and doesn't make some 645's smaller. There was an image comparison of a Contax 645 and a Bronica RF 645 and the SLR was big.
I can't really comment about 35mm vs 645, even though I have an ideals toward 645. All I have is taking a 6x9 frame and just do half
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