Sony A9 has a
24.0MP Full frame (35.6 x 23.8 mm ) sized Stacked BSI-CMOS sensor and features BIONZ X processor. On the other hand, Nikon Zf has a
25.0MP Full frame (35.9 x 23.9 mm ) sized BSI-CMOS sensor and features Expeed 7 processor.
Below you can see the A9 and Zf sensor size comparison.
Sensor Size and Resolution Comparison image of Sony A9 and Nikon Zf Cameras
Sony A9 and Nikon Zf 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
Sony A9
or the
Nikon Zf ,
including hyperfocal distance.