Sorry, but there is a relatively large difference in detail between the two. That has nothing to do with dynamic range, simply the pixel size. The D800 resolves quite a lot more detail than the 5D III. That either translates into the ability to enlarge more with the same level of detail as smaller enlargements with the 5D III, or it translates into naturally crisper, sharper images when downsampling for smaller prints or online publication.
"Large difference in detail" #1http://s11.postimg.org/kyh7iwp8j/5d3_d810_1.jpg
"Large difference in detail" #2http://s16.postimg.org/i7ha1whwl/5d3_d810_2.jpg
"Large difference in detail" #2http://s24.postimg.org/swj8ybket/5d3_d810_3.jpg
All images: left 5D mark III, right Nikon D810, DPReview studio scene RAW files at ISO 100 / NR0 converted using ACR standard settings. 5D3 file was resized to match (Bicubic Sharper) and then had light sharpening applied. I encourage everyone to download test files such as these and try them for yourself.
At best one can say tiny high contrast details are rendered a little better at pixel peeping sizes, and the D810 will sometimes pick up light, small patterns or textures missed on the 5D3. Shown at 50% on a 96 ppi monitor (i.e. 25x40" print) this stuff would be invisible. In print it would be invisible without a loupe.
We are well into diminishing returns for small format. People talk endlessly about AA filters and 22/24 vs. 36 MP yet they hardly matter at all even when pixel peeping tripod mounted shots made with primes at optimum apertures. Subject lighting, lens, technique, post processing, each 1,000x more important to fine detail and sharpness. I hope when Canon releases a high MP body they A) don't charge 1D prices, and B) shoot past 50 MP even if high ISO suffers. Because at this point I'm wondering if even >50 MP would yield truly significant differences in print without also jumping to a MF sized sensor.
This, btw, is why I'm going with the Sony A7 over the A7R. I see the exact same thing there. I would rather have electronic first curtain shutter, the phase detect points, and the extra cash in my pocket for lenses.Correction:
this is the Imaging Resource
studio scene (RAW / ISO 100 / NR0 / converted by me using ACR standard). I had these files on my HD already from when I was comparing 5D3 / A7 / A7R / D8x0. Sorry about that.