Film grain

Optical texture of processed photographic film

Film grain or film granularity is the random optical texture of processed photographic film due to the presence of small particles of a metallic silver, or dye clouds, developed from silver halide that have received enough photons. While film grain is a function of such particles (or dye clouds) it is not the same thing as such. It is an optical effect, the magnitude of which (amount of grain) depends on both the film stock and the definition at which it is observed. It can be objectionably noticeable in an over enlarged film photograph.

The size and morphology of the silver halide grains play crucial role in the image characteristics and exposure behavior. There is a tradeoff between the crystal size and light sensitivity (film speed); larger crystals have better chance to receive enough energy to flip them into developable state, as they have higher probability of receiving several photons needed for forming the Ag4 clusters that start the autocatalytic process of development. Large crystals will therefore give more sensitive film, for the price of being visibly grainier. Fine grain better preserves details but requires more light. Tabular grain film uses crystals of flat morphology, with width to thickness ratios of at least two, often much more. The flat morphology allows better overlapping of the crystals, reducing intergranular space and giving more black for the same amount of silver. The more compact structure allows for thinner emulsion layers. It is also more difficult to wash during the fixing stage. Tabular crystals also better absorb sensitizing dyes. They also scatter the light less, giving sharper image but less gradation. Tabular crystals also have less chance of absorbing high energy photons from ambient and cosmic radiation, giving longer shelf life without fogging. The tabular crystals can be favored during synthesis by an extra step, where the formed crystal seeds of undesired morphology are dissolved and the remaining ones grow by controlled Ostwald ripening. "Classical", cubic grain emulsion provides more random distribution of the crystal shapes and sizes, resulting in more "forgiving" film tolerant to wider range of exposures. Both morphologies can also be modified for a core shell structure, with a small silver halide grain being surrounded by one or more light capturing layers, or a more light sensitive center is surrounded by more developer sensitive shell. This gives finer grain for the same film speed. One of possibilities is a iodide rich core and iodide poor shell, giving high sensitivity to light inside and high sensitivity to developer outside. Both morphologies can also come in different distribution of sizes; "monosize", with narrow distribution of crystal dimensions, gives better control of the film speed and less visible grain (due to absence of larger crystals). Wider, more random size variation gives more tolerance to exposure (for too little light there are some big crystals, for too much light there are some little grains), and more tolerance to development process. Rod shaped grains, the opposite to tabular grains, can undergo self development even in absence of light, resulting in fogging.

Granularity, or RMS granularity, is a numerical quantification of density non uniformity, equal to the root mean square (rms) fluctuations in optical density, measured with a microdensitometer with a 0.048 mm (48 micrometre) diameter circular aperture, on a film area that has been exposed and normally developed to a mean density of 1.0 D (that is, it transmits 10% of light incident on it). Granularity is sometimes quoted as "diffuse RMS granularity times 1000", so that a film with granularity 10 means an rms density fluctuation of 0.010 in the standard aperture area. When the particles of silver are small, the standard aperture area measures an average of many particles, so the granularity is small. When the particles are large, fewer are averaged in the standard area, so there is a larger random fluctuation, and a higher granularity number.

Film grain is also sometimes quantified in a way that is relative independent of size of the aperture through which the microdensitometer measures it, using R. Selwyn's observation (known as Selwyn's law) that, for a not too small aperture, the product of RMS granularity and the square root of aperture area tends to be independent of the aperture size. The Selwyn granularity is defined as:

where σ is the RMS granularity and a is the aperture area.

Film grain overlay, sometimes referred to as "FGO", is a process in which film emulsion characteristics are overlaid using different levels of opacity onto a digital file. This process adds film grain characteristics, and in instances with moving images, subtle flicker to the more sterile looking digital medium. As opposed to computer plug ins, FGO is typically derived from actual film grain samples taken from film, shot against a gray card. Because film grain is difficult to encode because of its random nature, some video codecs, notably AV1, include film grain synthesis, where the film grain is removed during encoding and replaced with parameters that describe the shape and density of the particles, and during playback the decoder uses these parameters to resynthesize the film grain.