My first serious digital was the 1D Mark II I bought in 2005 which doubled the original 1D sensor size to 8 MP. In decent light, it created beautiful images but I shoot primarily sports and the sensor started getting "iffy" at anything above ISO 400. But I had zero complaints about its sharpness and detail when the lighting was good. With the 1DX III I can shoot and crop severely without concern at ISO 6,400 and it provides very good performance far above ISO 6,400 along with better dynamic range; that is where sensor and supporting hardware have really advanced over the life of the DSLR.
A camera is a system and the lens and user are most often the limiting factor, with sports and some wildlife (liked birds in flight) then the AF system also becomes a critical factor and is often the difference between a captured image and a miss. An inexperienced or ill-equipped owner can lean heavily on the camera technology but an astute photographer with a low end digital body will come away with better and more useful images than a dolt with a top end body and lens. The basics like positioning, composition, and other aspects of proper setup can sometimes be somewhat masked by the camera's "smarts" but not entirely ignored.
In terms of quality of output from the system, I would put the user as most critical, lens second, and image sensor along with its supporting hardware cast third. It is the reason I am still baffled by the "mirrorless images are better" crowd because with a DSLR the mirror is NOT in the light path during image capture. Mirrorless makes a better design for dual use video and still cameras which is the direction the world has gone but a sensor of a certain design and technology is a sensor whether it is always exposed or is covered by a mirror at times and when hit by light it captures the same image.
I will look at the R1 when it comes out and at some point I will likely be forced to move to mirrorless because that is where the new development dollars are going. Hopefully by then the focus performance will have caught up to the dedicated AF array of a DSLR, it seems the much vaunted gain by quad pixel mirrorless AF is to provide the same AF capability in both landscape and portrait orientation but the technology still lags what the top DSLR offernings from Canon and Nikon can do when rapidly grabbing focus between two widely spaced points on a field of play. That needs to be overcome because buying new to take a step backwards with a critical capability isn't of interest to me. The biggest difference I find shooting between my 1DX II and III bodies is an already great AF system got a little better and that was a valuable change.
Rodger
EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video
