What is Noise in the Digital Image?
Noise is just LACK of GAIN. Basically there are 2 types of gain in the DSLR. Native Gain & Applied Gain.
There are multiple types of native gain. Aperture, shutter speed & pixel pitch are the main native gain which CANNOT be change after you take the image. You can change these gains and increase of decrease the noise into the image. Every camera pixel pitch is based on its mega pixel count. Lower the mega pixel on full frame means bigger the pixel pitch which means better gain. The pixel can accommodate or capture more light and they are more sensitive to the light. When any sensor is manufactured, the pixel pitch is fixed and cannot be change. Same goes for the Aperture & Shutter Speed. Bigger aperture lenses like f/1.4 for primes or f/2.8 for zoom provides better gain. Slower shutter speed provide better gain and vice versa. All these parameters cannot be change AFTER you take any time.
ISO is applied gain which you can change BEFORE & AFTER taking the image. ISO is input gain, it has no connection to the actual exposure. As I mentioned above, Noise is lack of Gain. Noise is filtered by SNR firmware which is embedded into the main processor of the camera. Noise has about 6 or more frequencies that are dialed out in signal processing before the final output dumped onto a SD/CF card.
Happy Learning Guys !