In days of photographic films with silver halide emulsions, tonality was recorded when light struck sub-sand-size grains of silver coated in a gelatin emulsion on a plastic film base changing their molecular properties. When light was “exposed” to the silver grains on the film they were altered on a molecular level. But until the exposed silver grains were bathed in developer fluid, no evidence of an image was evident. This exposure induced change of properties was referred to as a “latent” image. The image was present, but not yet visible. If you were able to see the exposed film inside the camera, there would be absolutely no evidence of a physical change to the film.
However, when the film was transferred to a light-tight tank and chemical developer fluid did come in contact with the exposed silver, each grain was “cooked” (depending on the amount of light striking it), changing its density (light blocking ability) accordingly. The amount of time the film emulsion was left in contact with the developer fluid (and the current temperature of this developer fluid) changed these latent particles into “developed” particles. The longer the film was exposed to light (and cooked by the developing process), the more dense (and pronounced) each film grain became. When the development process was stopped (using an acid-based “stop bath”) and then processed with another fluid (hypo clearing agent, or “fixer”), the unexposed silver was rinsed from the film, rendering the film translucent. The end result was a film substrate with a negative image (charred silver where exposed) and clear film (where the silver was not exposed).
A little more background… Films were produced with different categories of silver grains, generally in three sizes. Coarse (Kodak’s Tri-X), Medium (Plus-X), and Fine (Panatomic-X). The more coarse the grain, the more defined and “sharp” the contrast of the picture. Coarse-grain films were also faster because the clumps of silver halide were more sensitive to light. Photographers loved to shoot Tri-X because it allowed them to capture moving objects and effectively “stop” the action. But the coarse grain films had a drawback… visual grain. The downside of the fine-grain films were that they were slower to expose and softer in appearance. Fast films were sharp but grainy and slow films were softer but able to be enlarged significantly before grain was visible.
There is a direct correlation between the film’s exposure and the developing solution’s influence on these silver grains. An image that was captured in a low-light situation required more exposure time to register that image properly on the silver halide particles. Images captured in abundant light require less time to properly expose. Sort of like filling a glass with water. Under a forceful stream of water a glass may fill up in a short amount of time. Turn the water volume down and the glass takes longer to fill.
However, there was a way to change the normal relationship between light and exposure time, but it came with a tradeoff side-effect. If the image was captured in low lighting conditions with insufficient lighting for a proper exposure, the development process could be enhanced to accommodate the insufficient exposure. One of two (or perhaps both) ways to increase the film’s silver density was to 1) heat-up the developing solution, exciting the chemical reaction process, or 2) increase the development time to more thoroughly cook all available exposed grains of silver. Both of these “pushed” development processes increased the visibility of the silver particles. This, in essence, was the origin of the term “grainy.” This photographic over exposure in development is akin to exposing nice pink skin to harsh sunlight for an extended period of time. Both behaviors yield the same result… scorch!
When underexposed and over-developed film is processed, the grains of silver become much more pronounced, as in “grainy.”
On the digital side of the fence, low lighting scenarios provide CCD and CMOS elements in digital camera image sensors with insufficient information to register accurately as tones. The image sensors in your camera are like little light meters… they simply record the amount of light present in a given scene. We call the darkest areas of scenes shadows. These shadow areas are dark because they lack sufficient light strength to be read accurately by camera CCD/CMOS array. (Image sensors are called digital film because they function much like their analog film counterpart. To counter this low light problem camera manufacturers offer faster lenses and more sensitive image sensors (higher ISO numbers).
On to the editing process… the attempt to lighten very dark shadow tones in Photoshop-type applications. When a very dark portion of the digital image is lightened (using either the Levels or Lighten Shadows dialog tools), there is often not enough “information” in the image to deliver meaningful (consistent) tonal value. Instead, unevenly exposed pixels deliver a “noisy” result. Noise is nothing more than sketchy electronic information. When asked to produce meaningful detail, these underexposed pixels deliver sandpaper-type irregular and erratic results. This digital noise is a direct result of insufficient scene lighting.
So what do we conclude? Low lighting causes pronounced and unattractive results from both analog film and digital cameras. In this way, digital noise and film graininess are quite similar issues; one purely physical in nature, one purely electronic in nature. The solution to the film grain problem was to either 1) add more light in the scene, 2) airbrushed prints, or 3) sell the client on a “special effect.” The solution for digital film is much more attractive… either more light (as with film), or shooting in RAW mode, capturing more information, or digital airbrushing using image editing software.