Both Canon 40D and Canon 400D 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 40D and 400D sensor size comparison.
Sensor Size and Resolution Comparison image of Canon 40D and Canon 400D Cameras
Canon 40D and Canon 400D 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
Both Canon 40D and Canon 400D sensors have been tested by DxoMark. DxoMark 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. Of the two cameras that we are comparing, 40D has scored 64, 2 points higher than 400D.
Model
Overall
Color Depth
Dynamic Range
Low-light ISO
Canon 40D
64
22.1 bits
11.3 Evs
703 ISO
Canon 400D
62
22.1 bits
11.0 Evs
664 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
Canon 40D
or the
Canon 400D ,
including hyperfocal distance.
