The photon counting model of photography:

By understanding just a little bit of physics, we can make better decisions about equipment and methods.Set 4 sm C

The image on the left was made at an ISO of 250 times that of the image on the right.  We see speckles of noise.  We see the loss of color definition.  We see the inability to resolve small details like the fur.  The subject on the right was sampled 250 times longer to increase the number of photons detected and therefore increase the information.

Light travels like a wave but is detected as singular events as if light was a particle.  Each of these particles (photons) provides information about the subject.  A single pixel in a dark area of an image might detect only 2 photons or 3 photons but it can never detect a fraction of a photon.  The result is quantum noise that we see as speckles.

From this simple concept we can draw some interesting conclusions: 

1. Efficiency is critical.  Sensor efficiency is the percentage of photons that hit the sensor that are detected by the electronics.  Present day cameras are many times more efficient than those of just a few years ago (and much better than film).  Future cameras can be several times better than today's but they will reach a limit when every photon is counted.
2. As sensor size increases the number of photons increase proportional to the area.  Small digital cameras have about 1/10 the area of the common DSLR.  Cell phones are even worse. 
3. Multiple images can be added to reduce random noise.  Although HDR can increase noise in highlights, noise can be reduced (not increased) in HDR by adding more exposures.  More on this in a later post.
4. Stereo Cameras (similar to stereo audio) provide twice the information about the subject.
5. Avoid Light field cameras (Lytro) and other methods that reduce or discard information.

I don't suggest that we all buy a medium format camera just to get a sensor that is twice as large as 35mm full frame.  Sensor efficiency, lens quality, convince, versatility, speed, weight, size and cost are also important. .

A more complete analysis of image quality can be found at http://www.clarkvision.com/.  The articles are filled with engineering details and good information.  A easy to understand description of photon counting can be found in Peter van den Hamer's article of December 15 2012 on DXOMark camera scores. 

  http://www.luminous-landscape.com/essays/dxomark_sensor_for_benchmarking_cameras2.shtml