I recently switched from an Aptus 65 to an Aptus 10 and find the 2x increase in resolution is pushing the limits of what I can humanly do to achieve accurate focus.
Doing the math the resolution increase is actually "only" a 51.921 % increase. It is very important to keep in mind that resolution is a linear function. MP count is a surface area
calculation. Resolution is described by lines per millimeter, not by lines per square millimeter.
With that in mind the percentage of resolution increase between a D800E and the Aptus-II 10 is only a 26.821 % increase.
That is calculated on using a full frame with both cameras. It is also quite easy and quick to do two shots with the D800 using lateral shift
with the tripod attached to the lens (with an adapter) and do stitches to get a wider format and more resolution.
Attaching the tripod to the lens means you move the camera not the lens for stitching. You can really stitch with close up subjects with significant subject depth if you move the lens.
Now with that in mind there is the issue of dynamic range. The Dynamic range of the D800E is higher than that of MFD Backs.
Personally I find that for print sizes of 40x60 you really have to look close to see a difference in resolution of about 25%.
However in natural light conditions with deep shadows and highlights more dynamic range is going to be more evident, especially if you are doing post to your images and using
color filtering in your black and white conversions.
In consideration of lenses... the Nikon 85mm tilt shift lens, the PC-E model is excellent. The 24mm is too. The 45mm, the oldest is due for a refresh.
You could also use the Schneider TS lenses.
I am pointing this out because you want or would like better live view. With the D800 you have a very high refresh rate with live view
and very quick navigation. You can also connect the camera to a PC tablet and get live view on a big screen, but being over USB it is not full res.
Then there is another option. You can connect to a field HDMI monitor. There are HDMI monitors with focus peaking, viewing hoods etc.
There is also a low budget HDMI option using a Motorola cell phone laptop screen and keyboard. Just plug the HDMI in and the live view
is displayed on the 11 inch screen. These can be found for under 100 bucks. Color quality is so so, but for focusing it's great.
Also seems to me that a small DSLR would be more practical in the field.
Here is an interesting comparison between the d800E Nikon PC-E 24mm tilt/shift lens and
the IQ180 back on an Arca Swiss Rm3d with a Schneider 35mm APO-Digitar XL
Their conclusion:
At 30×20 inches, you can see subtle but clear differences between the IQ180 and the D800E. Not all of them weighted in favour of the medium format camera, though. For instance, the D800E produced much more pleasing shadow areas on the prints of the photographs produced to test dynamic range.
Resolution and detail of the IQ180 prints was better than that of the D800E prints – but not massively. Again, the difference was there, but it wasn’t huge. Certainly not €30,000 huge.
And we were stunned just how close the D800E ran the IQ180 when the files were printed at 60×40 inches, which is bigger than many dining room tables.
Put simply, Nikon has produced a phenomenal camera.
Medium format camera manufacturers have cause for concern.
Here is the test.
http://www.circleofconfusion.ie/d800e-vs-phase-one-iq180/