Exposure Index (EI) is a term used to describe the effective sensitivity of a photographic film or digital sensor to light, essentially indicating how much light is needed to achieve a correct exposure. It is closely related to, and often used interchangeably with, film speed (ISO). Photographers may adjust the EI to compensate for underexposed or overexposed conditions, effectively "rating" the film higher or lower than its box speed to achieve desired results, such as pushing or pulling film during development.
Understanding exposure index is important for controlling exposure, planning film workflows, and predicting the tonal range and grain structure in final images. Whether working with traditional film or digital cameras, knowing the EI allows photographers to make informed decisions about sensitivity, lighting, and development processes, ensuring consistent and high-quality results under various shooting conditions.