As I know, for any lighting condition, there is a recommended/indicated/correct exposure value for the camera to render the object as middle gray.
The indicated exposure value can be calculated from the scene luminance and illuminance using the exposure equation described in Relationship of EV to lighting conditions .
If the calibration constants of two camera are the same, the indicated exposure values of these camera at the same lighting condition are the same. Canon and Nikon use the same value for these constants. Other manufactures use slightly different values.
For any lighting condition, the cameras with the same exposure value will receive the same exposure (i.e. the amount of light hitting the sensor measured in lux second).
So I guess the required exposures for all cameras to render the object as middle gray are the same (or slightly different in case the calibration constants are slightly different).
My first question: Is my guess right?
Since the exposure equations were used for film camera many years ago. I think this "required" exposure is quite universal for all cameras, whether they are film or digital.
My second question: What is the value of that "required" exposure (in lux second)?