Exposure is all about making sure the sensor gets the right amount of light. Without camera exposure controls, pictures taken at night would be pitch black, and those taken in bright daylight might be burned out. There is actually a fairly narrow ‘window’ of light intensities which sensors can record, and real-world lighting conditions vary over a much wider range.
This is why cameras have exposure controls to adjust the amount of light reaching the sensor. They also have light meters to measure the light in the scene to work out what the exposure should be.
Cameras have traditionally used two ways of controlling the exposure – by changing the length of time the sensor is exposed to light (the shutter speed) and by adjusting the intensity of that light (the lens aperture).
Shorter shutter speeds reduce the exposure, longer shutter speeds increase it. Smaller lens apertures reduce the exposure, wider lens apertures increase it.
Cameras use these two controls in combination to get the correct exposure. There are various technical and creative reasons why you might want to use a particular shutter speed or lens aperture, however, and this is why the idea of ‘exposure reciprocity’ is used.
Shutter speed and lens aperture values work on the basis of ‘stops’ (an old term, but still used) or EV (exposure values), which is the same thing but the more modern term. When someone says they’ve increased the exposure by ‘1 stop’, it’s the same as increasing it by 1EV.
Each ‘step’ in shutter speeds halves or doubles the exposure time and hence the exposure. Each ‘step’ in aperture settings does the same thing. This means that if you decide you want a faster shutter speed (which will reduce the exposure by 1 stop, or 1EV), you can compensate by increasing the aperture by 1 stop. That way, the exposure stays the same.
You can see this in action in different camera exposure modes. In Program AE mode the camera will choose a shutter speed and lens aperture automatically – but you can use the camera’s program shift mode to ‘shift’ the shutter speed towards a different setting and the camera will compensate by changing the aperture value. Similarly, if you want to shift the aperture setting to a different value, the camera will change the shutter speed to compensate. The overall exposure stays the same throughout. In Aperture priority mode you choose the lens aperture and the camera chooses a shutter speed to give the correct exposure. In Shutter priority mode, you choose the shutter speed and the camera sets the aperture. In Manual mode, the camera’s light meter will suggest shutter speed and lens aperture combinations but you’re free to choose these yourself, balancing shutter speed against lens aperture to get the correct exposure.
Digital cameras have added a third exposure control – the ISO (sensitivity) setting. With film, the ISO was part of the film’s characteristics. Once your film was loaded, your ISO was unchanging, unless you loaded a different film with a different ISO sensitivity.
With digital cameras, however, you can change the ISO setting at any time. This gives you a third exposure control, because you can change the ISO setting as well as the shutter speed and lens aperture. Again, ISO ‘steps’ will halve or double the exposure, so it’s quite easy to work out what ISO adjustments you need to balance other exposure settings.
In practice, most photographers still use shutter speed and lens aperture for everyday exposure control, and keep ISO adjustments for low light conditions where they may be needed to allow shutter speeds fast enough to avoid camera shake or freeze fast movement.
Nevertheless, ISO is now considered a third means of exposure control, and makes up what is now called the ‘exposure triangle’.