Measuring Light

Key Terms

Measuring the amount of light falling on a scene is critical  quality photography.   Once the quantity of light has been determined, it is possible to accurately set the f/stop openings and the shutter speed.

TTL Metering
    TTL stands for "through-the-lens".  This type of metering is built into SLR cameras.  Photocells measure the light and transmit it to a meter display visible through the viewfinder.  The exposure meter index informs the photographer whether the exposure settings are correct, over, or under exposed.

Manual Adjustments
    The following procedures can be used to manually set an SLR camera to accurately record the scene being photographed.

1. Select the scene
2. Pre-select the Aperture or shutter speed  (aperture pre-selection - Set the f/stop according to the scene and lighting conditions.  If greater depth of field is desired, a small aperture opening must be used.  If the lighting is low a larger aperture opening will need to be used) (shutter pre-selection - Set the shutter speed according to the conditions of the scene.  It will be necessary to use a fast shutter if you want to freeze the action)
3. Activate the metering system.
4. Look through the viewfinder and observe the exposure reading and adjust your camera accordingly.

Computer Aided Metering

    Many cameras with TTL metering contain micro-computers that measure the light and automatically make camera settings.  This includes setting the aperture opening or the shutter speed.  The central processing unit (CPU) functions as the brain of a camera's sophisticated electronic system.  Cameras that select either the aperture or shutter are considered to be automatic.  Those that select both are programmed.

Aperture and Shutter Priority

    Aperture priority cameras require that the photographer first sets the aperture (f/stop) according to the scene conditions.  From this point, the microcomputer uses the information from the TTL light meter and selects the correct shutter speed.

    Shutter priority cameras require just the reverse of the aperture priority cameras.  The photographer needs only select the shutter speed, focus the lens and squeeze the shutter.  The correct f/stop will be selected by the camera.

Programmed Cameras

    The photographer needs only to select the scene, focus the lens, and squeeze the shutter button.  The camera does the rest.

Manual Override

    Most of the automatic and programmed cameras have this valuable feature.  The manual mode permits the photographer to select both f/stop and shutter speed.

Specular Light – Light that travels in one direction, and creates a very hard light.

Diffused Light – Light that is reflected and travels in many directions, creating a very soft light.

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