GoPro Hero9 Black has a
23.0MP (1/2. ) sized CMOS sensor and features GP1 processor. On the other hand, Nikon P900 has a
16.0MP (1/2. ) sized CMOS sensor and features Expeed C2 processor.
GoPro Hero9 Black's sensor provides 7MP more than Nikon P900'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.
Below you can see the Hero9 Black and P900 sensor size comparison.
GoPro Hero9 Black and Nikon P900 have the same sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture.