Toronto. Ferd Hurter and Chuck Driffield worked together in England. They were known in glass plate and film days for the H&D Curve. In 1890, the two spent their spare time as amateur photographers. They gained recognition after publishing a way to accurately determine glass plate (film) speed and contrast based on their experiments.
Their H&D curve correlated negative exposure with glass/film density after development. Before this dynamic duo worked their magic, the speed of any photographic dry plate measurement was empirical only (a “by guess and by god” approach). They devoted their personal time to accurate sensitometry, gamma (contrast), latent image theory, etc.
Theirs was an age of photographic research, study and standardization necessary to make exposure of dry plates and film reproducible in the days when creation of sensitive materials, exposure, and development were no longer necessarily done by one experienced person in a very brief period of time during an afternoon.