7.17. Exposure value (EV)

The sensitivity of camera gear at autofocusing or determining correct exposure metering is rated in terms of EV—exposure value—for a given lens type and film speed or digital ISO.

Since the amount of light hitting the film or sensor is determined by exposure time (shutter speed) and lens aperture, EVs are simply combinations of shutter speeds and apertures that produce the same exposures. For example, f/4 at 1/30 sec has an EV of 9, which is the same EV as f/2 at 1/125 sec.