Editing Film speed (section)

===Determining film speed===
[[File:ISO6speedMethod.png|thumb|540px|ISO&nbsp;6:1993 method of determining speed for black-and-white film.]]
[[File:Amsterdam, Red Ligth District, Graffiti 1996.jpg|thumb|200px|Recording film 1000 ASA, Red Light District, Amsterdam, Graffiti 1996]]

Film speed is found from a plot of [[optical density]] vs. log of exposure for the film, known as the ''D''–log ''H'' curve or [[sensitometry|Hurter–Driffield]] curve. There typically are five regions in the curve: the base + fog, the toe, the linear region, the shoulder, and the overexposed region. For [[black-and-white]] [[negative film]], the "speed point" m is the point on the curve where density exceeds the base + fog density by 0.1 when the negative is developed so that a point n where the log of exposure is 1.3 units greater than the exposure at point m has a density 0.8 greater than the density at point m. The exposure ''H''<sub style="font-size: 70%'">m</sub>, in [[Lux second|lux-s]], is that for point m when the specified contrast condition is satisfied. The ISO arithmetic speed is determined from:

:<math>S = \frac {0.8\;\text{lx⋅s}} {H_\mathrm{m}}</math>

This value is then rounded to the nearest standard speed in Table&nbsp;1 of ISO&nbsp;6:1993.

Determining speed for color negative film is similar in concept but more complex because it involves separate curves for blue, green, and red. The film is processed according to the film manufacturer's recommendations rather than to a specified contrast. ISO speed for [[color reversal film]] is determined from the middle rather than the threshold of the curve; it again involves separate curves for blue, green, and red, and the film is processed according to the film manufacturer's recommendations.