Exposure to light
Exposure is the volume of light that strikes a photosensitive medium—this can be either film or a digital image sensor; this guide concentrates on digital photography—and the critical ingredient in determining image brightness. The total exposure received by the image sensor is determined by the light intensity—which is a factor of both the lens aperture and scene luminance—and the duration of that exposure. The following equation expresses this relationship:
Exposure = Intensity × Time
The aperture is a circular opening located inside the lens; it controls the intensity of light passing through it. The shutter is a timed-barrier situated to the fore of the image sensor; it controls the duration that light exposes the sensor. On the camera, the aperture and shutter are the only settings for controlling the intensity of light reaching the image sensor.
In addition to the aperture and shutter, most discussions of exposure will feature a third element, the ISO. Unlike the aperture and shutter, which are physical mechanisms, ISO is an electronic function that simulates changes to exposure. Much like changing exposure, the ISO affects the final brightness of your pictures; however, unlike adjusting exposure, the effect isn’t achieved by varying the amount of light striking the image sensor.
Although various combinations of aperture, shutter, and ISO settings can result in an image of equal brightness, the distinct values selected will impart their unique characteristics to the final picture. This tripartite relationship between aperture, shutter, and ISO is often called the Exposure Triangle; however, given what’s been established about exposure, the name is incorrect. A more appropriate name would be the Effective Exposure Triangle. This relationship is expressed as:
Effective Exposure = Intensity × Time × ISO
Names and definitions aside, it’s helpful to develop an intuitive understanding for each of the settings, since correct exposure isn’t exclusively about producing a picture of sufficient brightness, it’s also about balancing the variables at your disposal to achieve the desired look.