Camera exposure is defined as the total amount of light allowed to fall on the photographic image sensor of your digital camera during the process of taking a photograph.
Camera exposure is controlled in three ways:
- Shutter Speed
- ISO setting
What is Correct Exposure?
“Correct” camera exposure is defined as an exposure that achieves the effect the photographer intended. The purpose of exposure adjustment (in combination with lighting adjustment) is to control the amount of light from the subject that is allowed to fall on the image sensor, so that it falls into an appropriate region of the sensor’s characteristic curve and yields a “correct” or acceptable exposure.
What is Overexposure and Underexposure?
A camera exposure may be described as overexposed when it has a loss of highlight detail, that is, when the bright parts of an image are effectively all white, known as “blown out highlights” (or “clipped whites”).
A camera exposure may be described as underexposed when it has a loss of shadow detail, that is, the dark areas indistinguishable from black, known as “blocked up shadows” (or sometimes “crushed shadows,” “crushed blacks,” or “clipped blacks”).
Can Overexposure and Underexposure Be Intentional?
Yes. As the image to the right shows, these terms are technical ones rather than artistic judgments; an overexposed or underexposed exposure may be “correct”, in that it provides the effect that the photographer intended. Intentionally over- or under- exposing (relative to a standard or the camera’s automatic exposure) is casually referred to as “shooting to the right” or “shooting to the left”, respectively, as these shift the histogram of the image to the right or left.
The “Zone System” is another method of determining exposure and development combinations to achieve a greater tonality range over conventional methods by varying the contrast of the ‘film’ to fit the print contrast capability. Digital cameras can achieve similar results (high dynamic range) by combining several different exposures (varying only the shutter speeds) made in quick succession.
Today, most cameras automatically determine the correct exposure at the time of taking a photograph by using a built-in light meter, or multiple point meters interpreted by a built-in computer, see metering mode.
In manual mode, the photographer adjusts the lens aperture and/or shutter speed to achieve the desired exposure. Many photographers choose to control aperture and shutter independently because opening up the aperture increases exposure, but also decreases the depth of field, and a slower shutter increases camera exposure but also increases the opportunity for motion blur.
‘Manual’ camera exposure calculations may be based on some method of light metering with a working knowledge of exposure values, the APEX system and/or the Zone System.
A camera in automatic exposure (AE) mode automatically calculates and adjusts camera exposure settings in order to match (as closely as possible) the subject’s mid-tone to the mid-tone of the photograph. For most cameras this means using an on-board TTL exposure meter.
Aperture priority mode gives the photographer manual control of the aperture, whilst the camera automatically adjusts the shutter speed to achieve the exposure specified by the TTL meter. Shutter priority mode gives manual shutter control, with automatic aperture compensation. In each case, the actual exposure level is still determined by the camera’s exposure meter.
The purpose of an exposure meter is to estimate the subject’s mid-tone luminance and indicate the camera exposure settings required to record this as a mid-tone. In order to do this it has to make a number of assumptions which, under certain circumstances, will be wrong. If the exposure setting indicated by an exposure meter is taken as the “reference” exposure, the photographer may wish to deliberately overexpose or underexpose in order to compensate for known or anticipated metering inaccuracies.
Cameras with any kind of internal exposure meter usually feature an exposure compensation setting which is intended to allow the photographer to simply offset the exposure level from the internal meter’s estimate of appropriate exposure. Frequently calibrated in stops, also known as EV units, a “+1” exposure compensation setting indicates one stop more (twice as much) exposure and “–1” means one stop less (half as much) exposure.
Camera exposure compensation is particularly useful in combination with auto-exposure mode, as it allows the photographer to bias the exposure level without resorting to full manual exposure and losing the flexibility of auto exposure.