Olympus E-M5 III has a
20.0MP Four Thirds (17.4 x 13 mm ) sized MOS sensor and features TruePic VIII processor. On the other hand, Olympus E-PL9 has a
16.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features TruePic VIII processor.
Below you can see the E-M5 Mark III and E-PL9 sensor size comparison.
Sensor Size and Resolution Comparison image of Olympus E-M5 III and Olympus E-PL9 Cameras
Olympus E-M5 III and Olympus E-PL9 have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the 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
Olympus E-M5 III
or the
Olympus E-PL9 ,
including hyperfocal distance.