Perhaps, it is worth saying a bit more about noise in digital photography, because I don’t want to leave you with the impression that the only type of noise is the counting noise that we have been discussing.  Every element of an electronic device, like a digital camera, can generate noise.

For the kinds of detectors used in digital cameras, CCDs and CMOS detectors, there are, generally speaking, three types of noise:

1. Dark or thermal noise, which reflects that fact that above absolute zero (and we are far above absolute zero) random thermal motion of electrons can cause the electrons to act as if they we caused by photons.  Or said differently heat rather than light causes the electron to appear in the pixel well.
2. The kind of counting noise that we have been discussing.
3. Read-out noise – noise that results from the transfer of electrons between adjacent pixels as they are read out of the detector.  This transfer is often described as a bucket brigade, where water can spill between buckets as it is moved along.

There are two other forms of detector noise to consider. I think these might be better considered as kinds of background in your image, but never-the-less.  Both of these are randomly distributed on your detector, but always occur in the same positions. These are:

1. Random but permanent variations in pixel sensitivity including hot (bright) pixels or dead (black) pixels.
2. Random fuzzy areas on your detector due to dust.

Finally, noise in the amplifier or other camera electronics can add noise to your image or even add systematic patterns such as banding, seen at low intensity.  If the amplifier has a temporal oscillation to it, for instance, and because the image is always read out the same way, these will appear as spatial bands in the image.

We will have the opportunity to discuss noise more in the future.  For now, the important point is that there is noise.  Some noise gets amplified along with the signal as you change the ISO setting.  Finally, noise ultimately limits image sgarpness and resolution.