Exposure Value - Relationship of EV To Lighting Conditions

Relationship of EV To Lighting Conditions

The recommended f-number and exposure time for given lighting conditions and ISO speed are given by the exposure equation

where

• N is the relative aperture (f-number)
• t is the exposure time (“shutter speed”) in seconds
• L is the average scene luminance
• S is the ISO arithmetic speed
• K is the reflected-light meter calibration constant

Applied to the right-hand side of the exposure equation, exposure value is

Camera settings also can be determined from incident-light measurements, for which the exposure equation is

where

• E is the illuminance
• C is the incident-light meter calibration constant

In terms of exposure value, the right-hand side becomes

When applied to the left-hand side of the exposure equation, EV denotes actual combinations of camera settings; when applied to the right-hand side, EV denotes combinations of camera settings required to give the nominally “correct” exposure. The formal relationship of EV to luminance or illuminance has limitations. Although it usually works well for typical outdoor scenes in daylight, it is less applicable to scenes with highly atypical luminance distributions, such as city skylines at night. In such situations, the EV that will result in the best picture often is better determined by subjective evaluation of photographs than by formal consideration of luminance or illuminance.

For a given luminance and film speed, a greater EV results in less exposure, and for fixed exposure (i.e., fixed camera settings), a greater EV corresponds to greater luminance or illuminance.