Both Nikon D5000 and Pentax K-r 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 K-r sensor size comparison.
Sensor Size and Resolution Comparison image of Nikon D5000 and Pentax K-r Cameras
Nikon D5000 and Pentax K-r 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
At Camera Decision, we also look at Dxo Mark sensors when available as we think it is a good indicator of image quality.
Model
Overall
Color Depth
Dynamic Range
Low-light ISO
Nikon D5000
72
22.7 bits
12.5 Evs
868 ISO
Pentax K-r
72
22.9 bits
12.4 Evs
755 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
Pentax K-r ,
including hyperfocal distance.
