For an objective look at audio signal chain performance here is a chart of tests I did for my Digital Bolex, which does have very high quality preamps. Compared to most low end camcorders the preamp distortion is 1/10th as much, but mic level signal to noise is about the same. Why? A camera body is a hot inherently noisy electronic environment. Using a decent mixer for line level input to the camera significantly improves performance. All of this is independent of mic quality. Before any considerations of appropriate microphone choice, the recording chain quality looks like this:
1. Mic direct to camera: worst
2. External high quality mixer to camera: better
3. External high quality double system recorder: best

1. Mic direct to camera: worst
2. External high quality mixer to camera: better
3. External high quality double system recorder: best

