Nikon Z6 III has a
24.0MP Full frame (35.9 x 23.9 mm ) sized Partially-Stacked CMOS sensor and features Expeed 7 processor. On the other hand, Sony A9 III has a
24.6MP Full frame (35.6 x 23.8 mm ) sized Global Shutter sensor and features BIONZ XR processor.
One other difference between these two cameras that is worth mentioning is that Sony A9 III'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 Z6 Mark III and A9 Mark III sensor size comparison.
Sensor Size and Resolution Comparison image of Nikon Z6 III and Sony A9 III Cameras
Nikon Z6 III and Sony A9 III 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
Nikon Z6 III
or the
Sony A9 III ,
including hyperfocal distance.
