I think you mean this thread (leads directly to my post there) http://www.canonrumors.com/forum/index.php/topic,2261.msg48485.html#msg48485unfocused said:Aargh! Somewhere buried in the posts of the last week or so (I think) was a discussion of RAW files, native ISO, how digital cameras read the data, etc. etc. Now I can't find it.
The problem with (a large) underexposure is that all the pixels in the image will be registered, for example, with values between 0 and 10 (compared to the maximum 255). Now you increase the exposure in post and get values between 0 and 100, but the problem is that you don't fill all the values between 0 and 100, you actually get only 0, 10, 20, 30...
If however you properly expose an image on the sensor, noise will randomly fill the gaps and thusly create a smooth image - this is a form of dithering.
One other thing: by underexposure I mean relative to what you want when you process the image, not what the camera decides with its 18% gray algorithm.
Here are examples: http://theory.uchicago.edu/~ejm/pix/20d/tests/noise/noise-p3.html