[Essay] The Meaning of H&D

In countries where the English language prevails, particularly the United Kingdom and the United States, it is common to use the H&D numeric expressions to denote the sensitivity of dry plates to light speed. Currently, dry plates range approximately from 300 to 750. The H&D scale, established by the photographic pioneers Ferdinand Hurter and Vero Charles Driffield, dates back to their 1890 research on the density curves of black plates. The scale provides a method to measure plate sensitivity. By plotting the vertical lines of density curves against horizontal lines (relating to exposure times), it intersects the point of 1 (the symbol for the inertia of film sheets). Subsequently, the numerical value divided by the constant 34 yields the H&D number. For instance, a value of 0.07 translates to 340, or roughly 486, and sometimes 490. Although practical, this method may not always be accurate and comparable, leading to its modification by new systems, such as by Scheiner and Eder-Hecht.