Pentax ist DL2 has a
6.0MP APS-C (23.5 x 15.7 mm ) sized CCD sensor . On the other hand, Sigma DP1x has a
5.0MP APS-C (20.7 x 13.8 mm ) sized CMOS (Foveon X3) sensor and features True II processor.
Below you can see the ist DL2 and DP1x sensor size comparison.
Sensor Size and Resolution Comparison image of Pentax ist DL2 and Sigma DP1x Cameras
As seen above, Pentax ist DL2 has a 1.3x Larger sensor area than Sigma DP1x. 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.
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
Pentax ist DL2
or the
Sigma DP1x ,
including hyperfocal distance.