Both Nikon D800 and Nikon D810 have Full frame sized 36.0 MP resolution sensors so sensor size and resolution is not a differentiator between these two cameras.
One other difference between these two cameras that is worth mentioning is that Nikon D810's sensor doesn't have an anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moiré occurring in certain scenes.
Below you can see the D800 and D810 sensor size comparison.
Sensor Size and Resolution Comparison image of Nikon D800 and Nikon D810 Cameras
Nikon D800 and Nikon D810 have the same sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture.
DxOMark Sensor Scores
DxOMark is a benchmark that scientifically assesses image quality of camera sensors. It scores camera sensors for color depth (DXO Portrait), dynamic range (DXO Landscape) and low-light sensitivity (DXO Sports), and also gives them an overall score. Nikon D800 and Nikon D810 sensors have been tested by DxO and the results show that D810 has a better overall score of 97, 2 points higher compared to D800's score of 95.
Model
Overall
Color Depth
Dynamic Range
Low-light ISO
Nikon D800
95
25.3 bits
14.4 Evs
2853 ISO
Nikon D810
97
25.7 bits
14.8 Evs
2853 ISO
Sensor size sets the circle of confusion, so it drives how much depth of field each
of these bodies gives you at a given aperture. You can calculate it exactly for the
Nikon D800
or the
Nikon D810 ,
including hyperfocal distance.
