essential skills: digital photography in available light
TTL light meters
TTL or ‘through the lens’ light meters, built into cameras, measure the level of refl ected light prior
to exposure. They measure only the refl ected light from the subject matter within the framed
image. The meter averages or mixes the differing amounts of refl ected light within the framed
image, and indicates an average level of refl ected 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 viewfi nder. 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 midtone, 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 fi nal image produced would show the wall as having a midtone
(the same tone as a photographer’s 18% gray card).
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.
Centre-weighted TTL metering