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
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
Hm, in
lux-s, is that for point m when the specified contrast condition is satisfied. The ISO arithmetic speed is determined from:
This value is then rounded to the nearest standard speed in Table 1 of ISO 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.
发表于 2011-12-6 22:39
发表于 2011-12-7 01:37
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