Film grain

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.

The flat morphology allows better overlapping of the crystals, reducing intergranular space and giving more black for the same amount of silver.

Tabular crystals also have less chance of absorbing high energy photons from ambient and cosmic radiation, giving longer shelf life without fogging.

[2] "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.

When the particles are large, fewer are averaged in the standard area, so there is a larger random fluctuation, and a higher granularity number.

Instead, the ISO setting on a digital camera controls the gain of the electronic amplifier on the readout circuitry of the chip.

Ultimately, high ISO settings on a digital camera operating in low light conditions does result in a noisy image, but the visual appearance is somewhat different from traditional photographic film.

This process adds film grain characteristics, and in instances with moving images, subtle flicker to the more sterile looking digital medium.