Both Nikon D70 and Nikon D40 have APS-C sized 6.0 MP resolution sensors so sensor size and resolution is not a differentiator between these two cameras.
Below you can see the D70 and D40 sensor size comparison.
Sensor Size and Resolution Comparison image of Nikon D70 and Nikon D40 Cameras
Nikon D70 and Nikon D40 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 D70 and Nikon D40 sensors have been tested by DxO and the results show that D40 has a better overall score of 56, 6 points higher compared to D70's score of 50.
Model
Overall
Color Depth
Dynamic Range
Low-light ISO
Nikon D70
50
20.4 bits
10.3 Evs
529 ISO
Nikon D40
56
21.0 bits
11.0 Evs
561 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 D70
or the
Nikon D40 ,
including hyperfocal distance.
