Editing Film speed (section)

==Film sensitivity and grain==
{{Main|Film grain}}
[[File:Korn HP4 ISO1600.jpg|thumb|Grainy high-speed B&W film negative]]
The size of [[silver halide]] grains in the [[emulsion]] affects film sensitivity, which is related to [[film grain|granularity]] because larger grains give film greater sensitivity to light. Fine-grain film, such as film designed for portraiture or copying [[original camera negative]]s, is relatively insensitive, or "slow", because it requires brighter light or a longer exposure than a "fast" film. Fast films, used for photographing in low light or capturing high-speed motion, produce comparatively grainy images.

[[Eastman Kodak|Kodak]] has defined a "Print Grain Index" (PGI) to characterize film grain (color negative films only), based on perceptual [[just-noticeable difference]] of graininess in prints. They also define "granularity", a measurement of grain using an RMS measurement of density fluctuations in uniformly exposed film, measured with a microdensitometer with 48 micrometre aperture.<ref name="Kodak_2000_Print-Grain-Index"/> Granularity varies with exposure&nbsp;— underexposed film looks grainier than overexposed film.

===Marketing anomalies===
<!-- This section is linked from several places inside this article. Do not remove or rename this section heading without also updating the links. -->
Some high-speed black-and-white films, such as [[Ilford Delta]]&nbsp;3200, [[Kodak T-MAX|P3200&nbsp;T-Max]], and T-MAX&nbsp;P3200 are marketed with film speeds in excess of their true ISO speed as determined using the ISO testing method. According to the respective data sheets, the Ilford product is actually an ISO&nbsp;1000 film,<ref name="Ilford_2010_Delta3200"/> while the Kodak film's speed is nominally 800 to 1000&nbsp;ISO.<ref name="TMZ"/><ref name="new TMZ"/> The manufacturers do not indicate that the 3200 number is an ISO rating on their packaging.<ref name="Ilford_2006_Delta-3200"/> Kodak and Fuji also marketed E6 films designed for pushing (hence the "P" prefix), such as Ektachrome&nbsp;P800/1600 and Fujichrome&nbsp;P1600, both with a base speed of ISO&nbsp;400. The [[DX encoding|DX codes]] on the film cartridges indicate the marketed film speed (i.e. 3200), not the ISO speed, in order to automate shooting and development.