This is only a simple test/comparison showing the difference of resolution between these cameras.
The ISO for both was 200, both used the same 50mm f1.8 AF-D in AF, f4, Aperture priority mode.
I took many of the same images and checked them all, the results were almost always the same (sometimes worse). I originally tried my 50mm f1.2 AIs @ f4, focusing in live view at 100% but the images were no better and I felt the AF lens to be a fair choice since my eyes were no longer a factor. I'm not getting into the math here, if you crunch the numbers and convert the number of pixels per mm to lines per mm, the D750 is supposed to resolve about 68 l/mm and the D700 about 50 l/mm. But calculation and from what I've read the D810 with it's 36mp sensor should resolve to 80 l/mm. Using HCC film years back I was able to resolve 136 l/mm with the same lenses. Something to think about: the reason zoom lenses seem to be much better in the past is the sensors were not as good as film to show their faults. But film has grain and balances out.