Pixel Vignetting

Question

Can someone help me visually identify Pixel vignetting (such as the one described here: https://photographylife.com/what-is-vignetting#pixel-vignetting)

Pixel Vignetting:

Digital cameras also suffer from pixel vignetting. Compared to optical vignetting, this type of vignetting is only applicable to image sensors. Since digital sensors are flat, their pixels are all built the same way and face the same direction. Pixels in the center of the sensor receive light rays head on at 90 degrees, while pixels in the corner receive them at a slight angle. Because of this, the sensors in the corners will receive slightly less light compared to the center, causing pixel vignetting. Unfortunately, pixel vignetting cannot be cured by stopping down the lens, since it is purely the result of the angle at which light reaches individual pixels on the digital sensor.

How can I differentiate it from regular noise? Any visual examples would be immensely appreciated.

Answer 1

Pixels in the center of the sensor receive light rays head on at 90 degrees, while pixels in the corner receive them at a slight angle.

This part is not true... all areas of the objective element project rays to all parts of the image. I.e. the center of the sensor is also receiving rays at an angle from all of the periphery lens areas. And some rays at the sensor periphery are also "head on."

Unfortunately, pixel vignetting cannot be cured by stopping down the lens, since it is purely the result of the angle at which light reaches individual pixels on the digital sensor.

This part is also not accurate. It is true that some rays falling on the periphery of the image sensor have the greatest angle of incidence, but that is also mitigated by stopping down.

Pixel vignetting is essentially just mechanical vignetting at the pixel level due to the micro lens and recessed photosite (with front illuminated sensor).

This is more of a design consideration... I.e. if the lens' flange opening distance is shorter/wider then the overall design will have to compensate for larger angles of incidence at the sensor. It's really only a problem if it hasn't been taken into account; or if for some reason it cannot be compensated for fully... I suppose it's also a potential issue if one were to adapt a lens that is not part of the system design.

But there are design changes that can mitigate it; backside illuminated sensors to remove pixel well depth and blocking, oriented micro lenses to redirect rays, etc. I'm not aware of any way to identify pixel level vignetting when/if it occurs as a user; but it could be measured as part of the overall QE and pixel well readout (photon-electron conversion).