Both Nikon D5000 and Nikon D90 have APS-C sized 12.0 MP resolution sensors so sensor size and resolution is not a differentiator between these two cameras.
Below you can see the D5000 and D90 sensor size comparison.
Sensor Size and Resolution Comparison image of Nikon D5000 and Nikon D90 Cameras
Nikon D5000 and Nikon D90 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 D5000 and Nikon D90 sensors have been tested by DxO and the results show that D90 has a better overall score of 73, 1 points higher compared to D5000's score of 72.
Model
Overall
Color Depth
Dynamic Range
Low-light ISO
Nikon D5000
72
22.7 bits
12.5 Evs
868 ISO
Nikon D90
73
22.7 bits
12.5 Evs
977 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 D5000
or the
Nikon D90 ,
including hyperfocal distance.
