Both Nikon D3100 and Sony A290 have APS-C sized 14.0 MP resolution sensors so sensor size and resolution is not a differentiator between these two cameras.
Below you can see the D3100 and A290 sensor size comparison.
Sensor Size and Resolution Comparison image of Nikon D3100 and Sony A290 Cameras
Nikon D3100 and Sony A290 have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.
DxOMark Sensor Scores
Both Nikon D3100 and Sony A290 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, D3100 has scored 67, 1 points higher than A290.
Model
Overall
Color Depth
Dynamic Range
Low-light ISO
Nikon D3100
67
22.5 bits
11.3 Evs
919 ISO
Sony A290
66
22.6 bits
11.5 Evs
615 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 D3100
or the
Sony A290 ,
including hyperfocal distance.
