There is more to photography than just capturing an image. There are entire disciplines of science dedicated to it. You can quickly step out of the realm of a photographer and enter into the sciences of optics and physics. Light itself is an electromagnetic phenomenon, its energy. So instead of thinking about cameras in terms of taking pictures, let's look at them as instruments designed to record energy.
The film camera operates in a different manner then the digital camera. In film, the silver-halide crystals are sensitive to light. When a light source enters the camera through the lens, it strikes these crystals, which are in the emulsion layer of the film. Where light hits a crystal, the crystal changes structure and it clusters with other nearby silver-halide crystals that were also struck by light.
These clusters are still extremely tiny and form what is called a "latent" (invisible) image. To make this latent image appear, we "develop" the film by immersing the film in a chemical bath that converts the silver-halide clusters into black metallic silver clusters that form an image we can see.
The process is different for a digital camera. We must form an image on a CCD chip. A CCD chip has a myriad of dots of silicon, called pixels that are recorded on its surface. Just like the silver-halide crystals, these pixels are light sensitive. When struck by light, they change. Nevertheless, unlike silver-halide crystals, they don't transform chemically. They transform electrically. When struck by light, each pixel produces a tiny electric charge. This charge is what enables us to "read" a picture digitally.
The front end of the digital camera is comparatively identical to the front end of a traditional film camera. Light enters the camera through a lens that focuses the image, and the focused light continues into the camera until it strikes the CCD chip. Here is where the digital camera is different from the film camera.
Each minuscule point of light strikes a silicon pixel, which immediately responds to that light by producing a tiny electrical charge. This electrical charge is "read" by the circuitry in the camera and converted into a number. The strengths of this charge ranges from 1 to 256 that we refer to as the shades of gray. Once the image is stored on the CCD chip, the CCD chip must transfer the digital image to either a memory chip or to a floppy disk so the CCD chip can recycle itself or clear its memory for the next image.
The time between snapping the shutter and the time the digital camera is ready for the next photo is called the recycling time or the time it takes the CCD to transfer the digital image to another chip or disk and to erase the existing image, preparing the chip to receive a new image.
The digital process and the chemical process differ not only in the media used to store the image, but also in the manner that it responds to the focused light entering the lens. The film camera must have a chemical process occur for the image to be preserved and the digital camera uses an electrical process to store the image on a CCD chip digitally.
The theory that many ghost hunters believe is that the camera is picking up light that the human eye cannot perceive. The accuracy of this depends on the type of camera that is being used. In 35mm cameras this theory is incorrect, as the majority of modern film is designed to react primarily to the visible portion of the electromagnetic spectrum, just like the human eye.
There is some sensitivity to ultraviolet, but manufacturers try to minimize this by adding dyes to the emulsion and adding special coatings to the lens. What little UV light that might reach the film would have a bluish cast to it, not the ubiquitous whitish shapes found in most ghost photos. To photograph areas of invisible light, a 35mm camera must use a specialized film.
These are typically infrared films like KODAK High Speed Infrared Film. It is a high-speed film with moderately high contrast, sensitive to light and radiant energy to 900 nanometers (nm) in wavelength. However, these films have to be handled carefully and require some training to use them correctly.
With digital cameras the theory is more plausible. The widespread use of digital cameras, and the popular discovery that the image sensor, called a CCD, is sensitive to both ultraviolet and infrared, has caused a resurgence of interest in photography with invisible radiation. All the CCDs used in consumer digital cameras are also sensitive to ultraviolet.
This is why digital cameras are more prone to capturing "orbs" than 35mm film. A camera's flash is composed of mostly UV light and when dust or airborne particles float in front of then lens, the UV light from the flash bounces off of the particles and they are recorded by the CDD. Here is a very crude test of IR sensitivity for digital cameras:
Skeptics will claim that any sixth grader can use a photo-editing program, like Photoshop, to fake a digital image. If we go under these criteria, then any sixth grader who takes a basic photography class can fake a print by combining two or more negatives or by touching up the negative to create a false print. Negatives do not guarantee validity, as some skeptics would like to fool people into thinking.
Trick Photography is very specific about how to create photo tricks just as digital manipulation can be done to create a trick photo. Digital cameras also have a negative of sorts; it is contained within the photo itself. Its called and EXIF file and this file will tell you what type of camera took the photo along with other relevant data on exposure and focus.
If an editing program manipulated a photograph, each and every step will be documented in the EXIF as well. However, it is quite simple to erase the EXIF file itself, simply changing the name of the photo will do so. Granted, most us change the file names as it is practical and makes things allot easier for the Webmaster. Generally, it's a good idea to save the original disc with the EXIF files intact. You can download EXIF programs, just google it, but as with any kind of program, some are better than others. So you might want to browse around abit until you find one that you like.