Nikon D6 has a
21.0MP Full frame (35.9 x 23.9 mm ) sized CMOS sensor and features Expeed 6 processor. On the other hand, Nikon D3X has a
25.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor and features Expeed processor.
Below you can see the D6 and D3X sensor size comparison.
Sensor Size and Resolution Comparison image of Nikon D6 and Nikon D3X Cameras
Nikon D6 and Nikon D3X 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 D6
or the
Nikon D3X ,
including hyperfocal distance.