exposure
Semi-automatic
The disadvantage of a fully automatic or program 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 fi nding 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 fi eld and movement blur) but still ensures the
meter indicated exposure or ‘MIE’ is obtained automatically.
Aperture priority (aperture variable or 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 fi eld is usually a 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 fi eld
sacrifi ced (see the chapter ‘Creative Controls’).
Shutter priority (time variable or 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 brightly
illuminated subject matter. Movement blur may not be possible when using an image sensor set
to a high ISO in bright conditions.
Orien Harvey