How it works · 04
Exposure
How aperture, shutter speed and sensitivity trade against each other, what an exposure value is, and why old rules of thumb like 'sunny 16' still work.
Try it: the exposure triangle
- Scene brightness
- Settings (EV at ISO 100)
- Exposure
- The moving cyclist
Three controls, one result
A picture needs a certain amount of light per unit area of film or sensor: the exposure. Three settings decide it. The aperture sets how bright the image is; the shutter speed sets for how long it falls on the film; the ISO speed sets how much light the film or sensor needs. Each is marked in stops. Opening the lens one stop, doubling the exposure time or doubling the ISO all have the same effect on brightness, so they can be traded: f/8 at 1/125 s gives the same exposure as f/5.6 at 1/250 s or f/11 at 1/60 s.
What changes between those choices is everything else. A wide aperture gives shallow depth of field (see aperture); a slow shutter blurs movement; a high ISO adds grain on film and noise on a sensor.
Exposure value
Photographers sum up a combination of aperture and shutter speed in one number, the exposure value: EV = log₂(N²/t). f/1 at 1 s is EV 0; each step up halves the light. A scene can be described the same way: bright sun is about EV 15 at ISO 100, an overcast day about EV 12, a living room at night about EV 6. Match the camera’s EV to the scene’s and the exposure is right. The old “sunny 16” rule says the same thing in words: in bright sun, use f/16 and a shutter speed of 1 over the ISO.
Meters and automation
For a century, photographers judged exposure from tables, experience or a test plate. The selenium light meter of the 1930s measured light directly; through-the-lens metering in the 1960s measured it through the taking lens, and auto exposure let the camera set the shutter or aperture itself. A meter assumes the scene averages to a mid-grey, so snow and black cats fool it; Ansel Adams’s zone system taught photographers to place tones deliberately instead.
Reciprocity and its failure
In theory, halving the light and doubling the time always gives the same result. Film obeys this only over a middle range. In very long exposures (seconds to minutes) a silver grain struck by a few photons slowly loses its latent image before more arrive, so the film needs extra time: reciprocity failure. Sensors have no such problem, but they have their own limit: a pixel can hold only so many electrons, and the range between that and the noise floor is the sensor’s dynamic range.