Olympus E-PM1 has a
12.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features TruePic VI processor. On the other hand, Olympus E-PL8 has a
16.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features TruePic VII processor.
Below you can see the E-PM1 and E-PL8 sensor size comparison.
Sensor Size and Resolution Comparison image of Olympus E-PM1 and Olympus E-PL8 Cameras
Olympus E-PM1 and Olympus E-PL8 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 Olympus E-PM1 has 30% larger pixel area compared to Olympus E-PL8, 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
Olympus E-PM1
or the
Olympus E-PL8 ,
including hyperfocal distance.