Both Nikon D3000 and Canon 40D have APS-C sized 10.0 MP resolution sensors so sensor size and resolution is not a differentiator between these two cameras.
Below you can see the D3000 and 40D sensor size comparison.
Sensor Size and Resolution Comparison image of Nikon D3000 and Canon 40D Cameras
As seen above, Nikon D3000 has a 1.1x Larger sensor area than Canon 40D. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in 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 D3000 and Canon 40D sensors have been tested by DxO and the results show that 40D has a better overall score of 64, 2 points higher compared to D3000's score of 62.
Model
Overall
Color Depth
Dynamic Range
Low-light ISO
Nikon D3000
62
22.3 bits
11.1 Evs
563 ISO
Canon 40D
64
22.1 bits
11.3 Evs
703 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 D3000
or the
Canon 40D ,
including hyperfocal distance.
