Sony A7R V has a
61.0MP Full frame (35.8 x 23.8 mm ) sized BSI-CMOS sensor and features Bionz XR processor. On the other hand, Sony FX3 has a
12.0MP Full frame (35.6 x 23.8 mm ) sized BSI-CMOS sensor and features Bionz XR processor.
Sony A7R V's sensor provides 49MP more than Sony FX3's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Another difference between these two cameras is that Sony A7R V's sensor lacks 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 A7R V and FX3 sensor size comparison.
Sensor Size and Resolution Comparison image of Sony A7R V and Sony FX3 Cameras
Sony A7R V and Sony FX3 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 A7R V
or the
Sony FX3 ,
including hyperfocal distance.