TTL light meters

TTL or ‘Th rough-the-lens’ light meters, built into cameras, measure the level of refl ected light prior to exposure. Th ey measure only the refl ected light from the subject matter within the framed image. Th e meter averages or mixes the diff ering amounts of refl ected light within the framed image, and indicates an average level of refl ected light. Th e 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. Th e 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. Th is 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 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. Th is 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.

The disadvantage of a fully automatic or programme mode is it can often take away the creative input the photographer can make to many of the shots. A camera set to fully automatic is programmed to make decisions not necessarily correct for every situation. If your camera is selecting both the aperture and shutter speed you will need to spend some time finding out how the camera can be switched to semi-automatic and manual operation. Semi-automatic exposure control, whether aperture priority (Av) or shutter priority (Tv), allows creative input from the photographer (depth of field and movement blur) but still ensures MIE is obtained automatically.

Aperture priority (Av)

This is a semi-automatic function where the photographer chooses the aperture and the camera selects the shutter speed to achieve MIE. This is the most common semi-automatic function used by professional photographers as the depth of field is usually the primary consideration. The photographer using aperture priority needs to be aware of slow shutter speeds being selected by the automatic function of the camera when selecting small apertures in low-light conditions. To avoid camera shake and unintended blur the aperture has to be opened and the depth of field sacrificed.

Kata Bayer

Shutter priority (Tv)

This is a semi-automatic function where the photographer chooses the shutter speed and the camera selects the aperture to achieve correct exposure. In choosing a fast shutter speed the photographer needs to be aware of underexposure as light levels decrease. The fastest shutter speed possible is often limited by the maximum aperture of the lens. In choosing a slow shutter speed the photographer needs to be aware of overexposure when photographing a brightly lit subject. Movement blur may not be possible when using a high ISO in bright conditions.