What is ISO (film speed)?
ISO (film speed) meaning in Digital terms is Used colloquially in the context of film photography, ISO followed by a number (e.g., 400) represented the sensitivity of a given film emulsion to light, often referred to as "film speed." Higher ISO numbers indicated a greater sensitivity to light. The emulsion speed sensitivity was determined by the standards of the International Standards Organization (ISO), which is how the term ISO came to be used in this context. In digital cameras and scanners, the image sensor has a fixed sensitivity or response to light, but the colloquial ISO is still used in a similar manner as with film. When changing the ISO on a digital imaging device, the gain is changed rather than the sensitivity of the image sensor. Increasing the gain increases the signal amplification from the sensor making it appear to be more sensitive. Increasing ISO on a digital camera or scanner increases the noise relative to the signal, decreasing the signal-to-noise ratio (SNR).