Speed in photography refers to the sensitivity of a photographic emulsion to light, indicating how quickly the film or sensor reacts when exposed. A material with a higher speed requires less light to produce a correctly exposed image, while a slower speed requires more light or longer exposure times. Film speeds are commonly rated using ISO numbers, and historically DIN numbers, to identify their sensitivity characteristics.
Photographic speed is an important factor when choosing film or camera settings, as it affects exposure, image quality, and shooting flexibility. High-speed films are ideal for low-light situations, fast-moving subjects, or handheld photography, but may produce more visible grain. Lower-speed films generally provide finer detail, smoother tones, and better image quality, making them popular for studio, landscape, and well-lit photography where maximum sharpness is desired.