Context: 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:

Question: What is the plot from which film speed is derived called?
Answer: the D–log H curve or Hurter–Driffield curve

Question: How many regions does the Hurter-Driffield curve have?
Answer: five

Question: What is plotted in the Hurter-Driffield curve?
Answer: a plot of optical density vs. log of exposure for the film

Question: What is denoted when the specified contrast condition is met?
Answer: The exposure Hm, in lux-s

Question: What are the five regions of the Hurter-Driffield curve?
Answer: the base + fog, the toe, the linear region, the shoulder, and the overexposed region

Question: What is film speed combined with to produce optical density?
Answer: Unanswerable

Question: What is a plot of film speed vs. log of exposure called?
Answer: Unanswerable

Question: How many speed points are there in a black and white film?
Answer: Unanswerable

Question: What is the point at which density is 0.1 less than the base + fog?
Answer: Unanswerable

Question: What is the ISO arithmetic speed?
Answer: Unanswerable