With the possible arrival of a 36-megapixel Sony NEX A7r soon, one might ask whether a high-res EVF could make it the camera of choice for manual focus DSLR lenses for many situations. Assuming the ergonomics are solid.
That is, for manual focus accuracy, vision limitations, leveling the camera while shooting handheld, for the sheer visual pleasure, for perfect 100% framing and so on. See the two references at top.
Piling on, DSLRs today are poorly suited to manual focus lenses—
- The optical viewfinder (OVF) often has limited coverage and sometimes is not 100% accurate or even level to the sensor.
- The focusing screens do not support fast lenses: f/1.4 and f/2 lenses cannot be “seen” properly; effectively focusing is happening at f/2.8 or so (it has to do with the bundle of rays and the glass in the focusing screen).
- The optical path to the OVF is separate from the optical path to the sensor and very likely to be off by at least 20 microns if not 40 microns or more. That amount is small physically, but means blurred results. And it is the difference between world-class performance and mediocre performance (or worse).
In short, the OVF is really only a framing device. The poster child for an OVF being problematic is the new Zeiss Otus 55mm f/1.4 APO-Distagon (because it is f/1.4 and also a very high performance lens): the Nikon D800E focusing screen is a severe handicap in obtaining precise focus for it (or any high quality f/1.4 lens).
It is not an either/or choice, or mirrorless vs DSLR choice
The real issue is having a camera get out of the way: smaller and lighter for carry comfort yet with high-grade image quality, a high-resolution EVF that allows precise manual focus (zooming in, focus peaking and exact framing and leveling). This sort of stuff dovetails into the keeper rate and efficiency and fun factors, a point driven home to me when shooting the the Sony RX1R for thousands of frames in the field: I did not want to put it down and I did not want to pick up my DSLR instead.
DSLRs are failing miserably in those regards.
Inertial thinking is involved here, with the design challenge being not violating the existing DSLR paradigm but enhancing it. And it is why Canon and Nikon are skating on thin ice: forcing buyers (like myself) to put up with arbitrary usability limitations for no technical reason is a serious danger to the C/N hegemony.
There is no reason why a DSLR cannot support a high quality add-on EVF in the hot shoe area, in addition to the OVF and the existing hot shoe. This is not a hard technical problem.