Please explain how the sensor does affect the light gathering capability of the lens. This is going to be very interesting. 😀
I have done an experiment.
I have a Nikon D300, EP1, G1, and and Epson RD-1, a Mamiya ZD, a light meter.
When you take a meter, measure the ambient, or use it to measure flash, I have done both; within a bit of latitude, when lenses are sent to the same aperture, ISO on the sensors is set the same, and shutter speed is set the same, the image is exposed the same. While I have not done it with a full sense of rigor, the results hold in practice.
I also shoot a 4x5 with both film and FujiRoid film, and MF format film. I do this in the studio with flash all the time. The ISO of my film and base iso of may sensors is all over the place. I keep the light the same, and only vary the aperture my exposure comes out right as long as I set the F stop to match the ISO ( I have been know to mess up the conversions, but that is a different story )
I'm not going to say equal, because they are not indeed equal, but for all practical purposes, they are the same. If there is enough force of voice out there, I'd be happy to run the experiment with a little more vigor.
Exposure has nothing to do with sensor size/sensitivity ( once calibrated ) is my practical conclusion. I would surely see it in 4X5 film vs APS-c digital, and I don't. ISO is a standard, and more or less, the manufactures calibrate to it.
In addition, maximum or minimum lens aperture made any difference, I would see differences between my F1.2 Nikon and my F5.6 Schnieder, and I found no difference in exposure. When set to the same value.
Of course DOF is a whole different kettle of fish.
Dave