essential skills: photographic lighting
TTL light meters TTL or ‘Through-the-lens’ light meters, built into cameras, measure the level of reflected light prior to exposure. They measure only the reflected light from the subject matter within the framed image. The meter averages or mixes the differing amounts of reflected light within the framed image, and indicates an average level of reflected light. The light meter readings are translated by the camera’s CPU and used to set aperture and/or shutter speed.
Centre-weighted and matrix metering Centre-weighted and selective metering systems (matrix metering), common in many cameras, bias the information collected from the framed area in a variety of ways. Centre-weighted metering takes a greater percentage of the information from the central area of the viewfinder. The reading, no matter how sophisticated, is still an average - indicating one exposure value only. Any single tone recorded by the photographer using a TTL reading will reproduce as a mid-tone, no matter how dark or light the tone or level of illumination. This tone is the midpoint between black and white. If the photographer takes a photograph of a black or white wall and uses the indicated meter reading to set the exposure, the final image produced would show the wall as having a mid-tone (the same tone as a photographer’s 18% gray card).
Centre-weighted TTL metering
Automatic TTL exposure modes If the camera is set to fully automatic or program mode both the shutter speed and aperture will be set automatically, ensuring an average exposure in response to the level of light recorded by the meter. In low light the photographer using the program mode should be aware of the shutter speed being used to achieve this exposure. As the lens aperture reaches its widest setting the program mode will start to use shutter speeds slower than those usually recommended to avoid camera shake. Many cameras alert the photographer to this using an audible or visual signal. This should not be treated as a signal to stop taking photographs but to take precautions to avoid camera shake, such as bracing the camera or by using a tripod.
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