OK here is another data point. Again please accept that this is for now somewhat unscientific, I am just playing and the weather is waaay too bad here to go do this outside with mid-distance building facades or landscapes on a tripod etc but I wanted to try a little handheld, real-world experiment. So I shot the same interior flat facade with the EM-1 with 12-40 and the D800E with 28-70. Both frames were shot at the widest end, wide open, both with centre single AF. But I set the Nikon to Auto ISO and in the menu setting, 'faster' was selected for the choice of shutter speed: in my experience, with a heavy and unstabilised lens like this, that is required if one is to reliably avoid camera shake. The Oly I set to ISO 200 and just let the shutter speed fall where it would, which was at 1/20th. Again, real world, that is how I would shoot a static subject with this setup.
Metering was auto for both and though the Nikon looks like it had a +1/3rd that is because my camera is permanently set to a -1/3rd bias. Nonetheless the Nikon gave the scene a bit more exposure, I estimate that at about a third of a stop, which is why I usually have it set to a small negative.
Then I gave both files the exact same LR settings, WB off the white card on the fridge (the Oly got the AWB correct, the Nikon did not). The only difference was I enabled lens corrections in the Nikon so as to match the in-camera adjustments made to the RAW file by the Oly. Nonetheless, the Oly had more vignetting but not to a degree that I mind about.
The Nikon chose an ISO of 1400 in order to get the shutter speed it felt it needed (1/100th). I could possibly have shot it at ISO 1000 instead with the same shutter speed so as to more closely match the exposure of the Oly frame but, the point of this was 'shoot them as you'd use them'.
Then I exported both files to 2304 pixels on the long side. That equates to a 200 DPI print of around 20" if you view the file at 100% on a normal (non-retina) screen. It also has the advantage of meaning that both files have been downsized from 'native' and therefore both are 'processed'.
Like I say, unscientific but interesting. It tends to confirm my suspicion that the Oly lens is sharper, especially at the edges, and that the noise advantage of the Nikon is largely lost due to the need to shoot at higher ISO, because there's no IS. Or course, in good light the Nikon would win much more clearly on file quality but for this print size, the difference would IMHO be more than negated by the better performance of the Oly lens.
Links are to the files.
http://tashley1.zenfolio.com/img/s10/v105/p1046503986.jpg
http://tashley1.zenfolio.com/img/s5/v128/p673561323.jpg
I will run tests such as this much more scientifically when time and weather allow but for now, this does tend to confirm my first impression of better wide end sharpness of the Oly lens.
