Sony A55 has a
16.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features Bionz processor. On the other hand, Pentax K-S2 has a
20.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features PRIME MII processor.
One other difference between these two cameras that is worth mentioning is that Pentax K-S2's sensor doesn't have an anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moiré occurring in certain scenes.
Below you can see the A55 and K-S2 sensor size comparison.
Sensor Size and Resolution Comparison image of Sony A55 and Pentax K-S2 Cameras
Sony A55 and Pentax K-S2 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. On the other hand, since Sony A55 has 24% larger pixel area compared to Pentax K-S2, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.
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
Sony A55
or the
Pentax K-S2 ,
including hyperfocal distance.
