The camera itself is not the real marvel in this process, a simple shoe box could be used as a camera with a pin hole poked to allow in enough light to expose a piece of film. The true marvel is taking that exposed piece of film and getting a picture from it.

Taking a picture with a film camera is a chemical and mechanical process (though many new cameras use elecronics, the final process is still mechanical). A lever with a plate covers the inside of the lens keeping all light from the inside chamber of the camera. The mechanisim works much like a gun, the spring loaded lever is cocked into place, when the shutter button is pushed, the plate lifts, light is let through the lens for a split second, then the spring drops the plate back into position over the lens keeping light out. (There are different variations of this process, but they all work in the same manner). During the time the plate is allowing light into the chamber,millions of little grains of silver halide are exposed to the light coming through the lens, These grains react to the light and change their chemical state. When taking a role of film for developing, these grains are dipped in several different chemicals to stabilize and stop the chemical reaction. Then washed to create the negative image. The negative is then used with photosensative paper and more chemical processing to create the final product. That is as simple an explination as I can give for photo processing, of course there is many steps involved, but for the purpose of this information, the fact that film photography is a mechanical/chemical process is what we really need to know.

This is a simple process, but there are still ways that this process can malfuntion and cause problems with the final pictures:

Light leaks from abused cameras, misaligned lenses and film doors not closed properly.
Intermittent shutter problems with older and dirty cameras, may hang once ever 20 pictures.
Mechanical shutter problems with electric drive cameras and weak batteries.
Moisture build up in the camera which reaches the film or condenses on the inside of the camera lens.
Internal springs, metal parts that the black covering wears off exposing the shiny metal underneath.
Blind style shutters misaligning and getting hung up intermittenly.
Image corruption during film processing.
Old film being used.
Heat causing problems with the film.

Since film has been around much longer then any of the other medias, there are a ton of theories to go with how a film camera can photograph paranormal activity. The first and probably the most predominent theory is that the film is actually picking up the spirits energy. Since photography is a chemical process, this is very difficult for me to understand. But once again, when dealing with the paranormal, the standard rules do not apply. Some religions believe that a picture actually steals part of the soul, so if spirit energy is actually the soul of a dead person, then this may very well be possible.

A film camera works much like the human eye, but the camera does not have that big grey lump attached to it processing all of the images that we see. The camera is going to record what is in front of it without any discrimination of what it is seeing. As a child we are told on a constant basis that ghosts do not exist, is it possible that we may have trained our brains not to register the events that the camera is picking up?