Doesn't that depend on lens choice?
You used the 15-30 and 24-70 on the 5D, but could have had much the same coverage by using a 10-22 and 17-55 on the 20D.
You used the 70-200 and 35 f/2 on the 20D, but could have had much the same coverage by using a 70-300 and 50 f/1.4 on the 5D.
My choice is to stick to one system and buy the lenses that work best with that system.
Well, the 10-22 doesn't go as wide on the 20D as the 15-30 does on the 5D. And I found myself using every bit of that width at times. AAMOF you just can't get as wide coverage on the APS-C as you can on the FF in either rectilinear or fisheye. And there isn't a 70-300 f/2.8 IS out there. I had 1.4x and 2x TCs along as well, and needed every bit of them too; wouldn't have had AF with the 70-300s that are out there. I get bodies for the same reason I get lenses - I assess their strengths as they fit into my shooting needs, and choose the mix that'll do me best for all-round work. If that means different sensor formats, no biggie. I want dual bodies for that once-in-a-lifetime trip anyhow. If anything (including one of the lenses) had failed, I would only have lost a little bit at one end of things.




