Visible light is different, because each photon has a lot more energy than in the 2.3GHz range. Your average decent consumer-level camera has a sensor that can nominally just about detect single photons some proportion of the time (as in, some of them bounce off instead of being detected) though it can't technically count them. The graininess on digital camera images is more from the Poisson noise of the incoming photons than it is from the applied noise of the sensor itself.