Film speed is found from a plot of optical density vs. log of exposure for the film, known as the Dlog H curve or HurterDriffield 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:

Answer this question, if possible (if impossible, reply "unanswerable"): How many regions does the Hurter-Driffield curve have?
five