Sometimes the left side of an image has a different brightness on left and right side or in one corner.
This effect doesn’t change if you turn the lens a little and also is unchanged if you turn the camera 180 degrees around its axis.
Most sensors have an unsymmetrical pixel structure with the light sensitive are in a corner or on one side of the pixel:
If the light arrives in allowed directions (the CRA is in tolerance), then usually all light arrives at the light sensitive part of the sensor pixel. An too low F# means that angles larger than the allowed CRA might occur and can cause troubles.
Is the Angle of the arriving light is larger than allowed, (or smaller than allowed for image side telecentric lenses) then some light will arrive in the neighbour pixels (consisting of a light sensitive part and of electronics to transport the charge).
Because the pixel structure is not symmetrical, light arriving from (for example) left will arrive in the “blind spot” of the neighbor pixel, while light from the other side will arrive in the light sensitive part of the neighbor pixel.
The local unsymmetry of the pixel structure leads to global unsymmetry of the image brightness!