The slope of the H&D curve is described by which descriptor?

• A. Average gradient
• B. Gradient
• C. Long scale contrast
• D. Photoelectric interactions

Answer: (B)

The slope of the Hurter-Driffield curve is described by the gradient. The H&D curve plots density against log exposure, and the gradient measures how quickly density changes as exposure changes. A steeper gradient means a rapid density increase for small exposure changes, which corresponds to higher image contrast (short-scale). A gentler gradient means lower contrast (long-scale). The term “average gradient” isn’t the standard descriptor for the whole curve’s slope, though you could compute an average over a region; the typical label is simply gradient. Long-scale contrast describes the visual result of a shallow slope, not the slope itself, and photoelectric interactions are a physical process, not a descriptor of the curve’s slope.