Grains and Noises
Both film grains and digital noises when in excess are detrimental to image quality, but the former often appears more natural and is even regarded by some as a much cherished property of film photography. Grains in aptly exposed films add a touch of softness that can be aesthetically pleasing to the eyes, but digital noises are undesirable artifacts that people want to get rid of, probably comparable with that squeaky violin notes that disrupts the harmony of the concert. You seldom hear people say that they want to keep a little bit of the noises to make their digital photos better. I believe most will go for noise reduction instead.
Grains as the inherent texture of the chemical film
Film consists of a photosensitive layer of silver halides, the ionic compounds formed by silver and a halogen, e.g., silver chloride, silver iodide and silver bromide, suspended in a gelatin emulsion. As a photon of light strikes, it knocks out an extra electron from a halide, and the electron reduces the silver ion into the metallic silver. Deposition of the inert silver atoms within the gelatin emulsion will then form the latent image. In other words, silver halide grains are the basic physical units that form the image on a film photograph.
Films with larger grains have a higher ISO value, i.e. photosensitivity, and are useful in dimmer environment, but often result in a more grainy texture. Films with finer grains have lower ISO and require more light for exposure, but are capable of creating sharper images that capture finer details.
Films with different ISO have different overall grain sizes. But it is important to note that in any given film, the silver crystals are not exactly identical in size and their distribution not a perfect lattice. There is still some variations and non-uniformity within the film itself. Underexposed films often appear more grainy because it is the larger grains that are getting exposed. The smaller, unexposed ones are washed away in the developing process. That is, larger silver crystals are more light-sensitive and they get to be exposed first in low light environment, giving rise to the enhanced graininess in underexposed films.
Noises as the signal interference from the electronic image sensor
A pixel is the fundamental unit of digital photography. It contains numerical values encoding information about a visual scene to be represented on a digital photo. In other words, a digital photo is a matrix of numbers representing the visual scene, in terms of intensity of colors i.e. the primary red, blue, green, and luminosity i.e. level of brightness.
A digital image sensor is a grid of pixels that receives and translates the incoming light into values that represent the intensity of red, blue, and green falling on each particular pixel, hence we have what we call the ‘RBG’ color model. Luminosity is the weighted average of the three color values, representing the overall brightness.
Digital noises appear as variable speckles across the image as a result of random signal fluctuations that the sensor perceives and translates into numerical values that are inaccurate representation of the actual scene you want to capture. Noises are manifested as random color shifts or variability in brightness due to inaccuracy in signal capture and interpretation.
I have tried to incorporate some digital noises into a photo that I took with Ektar 100. Perhaps not the fairest comparison, as the image on the right is edited to look like digital rather than actually taken with a digital camera, but I hope this can help to illustrate the textural difference between grains and noises.
