bobn2
Forum Pro
I think that a lot of these 'tradeoffs' are to do with your approach and thinking as to what 'ISO' and 'exposure' are. Now you're a multi-format user, time to embrace equivalence, it can really help you out in these situations.Shooting Tradeoffs
I am finding that the two cameras, OMDs and the d750, are not better or worse, just different.
* With the OMDs, I worry a lot more about how the background works with the subject; with the d750 I worry about whether I have enough DOF to convey the subject.
* With the OMDs, it's easier to nail the exposure because I can see it while I am shooting. With the d750, I am more likely to miss the correct exposure, but I have more latitude to clean it up in post.
* With the OMDs, I worry about whether the ISO is too high; with the d750, I worry more about whether my shutter speed is high enough. For example, the other day I set up my EM5ii with a 45mm f/1.8 and the d750 with an 85mm f/1.8. I set the aperture to f/2.5 on both camera, and shot the same thing at a range of shutter/ISO combinations. At 1/60 ISO 400, the EM5ii produced a nice, sharp picture while the d750 produced a blurry mess. At 1/20 of a second, the d750 was a disaster. At ISO 6400, though, the d750 looked great and the EM5ii was drowning in noise. My takeaway is to never underestimate what IBIS can do. It's easy to forget what a difference it makes.
For instance, you say that you 'worry about whether I have enough DOF to convey the subject' on the D750. Well, you can always stop down, that's what the aperture control is for. Of course, once you stop down, you won't be getting any better image quality from the D750, but that's the basic point, compared to mFT cameras, FF cameras are trading DOF for overall light, and therefore image quality. Similarly, no need to worry about where the 'ISO is too high' on the OMD. On neither of these cameras does the ISO control do a whole load to image quality it just sets the processing for the exposure you're using. If you have the same DOF and shutter speed on each camera, you will get pretty much identical image quality. So, you also don't need to worry about whether your shutter speed is high enough on the D750.
Whatever you can do with the EM-1, you can also do with the D750, you just need to set the ISO and f-number two stops higher and the shutter speed the same. What that tells you is that over most of its shooting envelope the D750 buys you nothing whatsoever over the EM-1 (well 4 extra MP, but that's not significant). Where it buys you something is when shutter speed is constrained, the ability to trade DOF for light, because it can go to shallow DOFs that the EM-1 can't. Even the other FF advantage, the ability to trade shutter speed for light doesn't apply much, because the EM-1 has high res mode which for static subjects also does this.
So, work out over which part of the shooting envelope the cameras are equivalent, then you know where the D750 is buying you something for the extra size and weight. (not so much for the D750, it's about the smallest FF DSLR there is)