Earlier quoted context omitted.
No, the intensity of light is not reduced. ISO is not gain. In other words, ISO 200 on full frame is the same as ISO 100 on M4/3 in terms of what actually happens inside the sensor. This is because of how the ISO standard is written. As for light angle, it's a consequence of the second law of thermodynamics, that translates in optics as the conservation of étendue. Basically what this means it that the amount of ligh…
The intensity of the light is reduced because the aperture is the same diameter (50/3.4 = 25/1.7), but the light has to cover four times the area. To get the same exposure with double the f number and the same shutter speed will require four times the ISO. > what determines in the same scene how much light is received by the sensor depends on the size of the aperture in milimiters That's true, but the amount of light…
Yes, you will need four times the ISO. But you will get exactly the same amount of noise.