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Wednesday, 4 May 2011

Measuring Brightness

Camera sensors are NOT as good as our eyes. There is a big difference in the range or more accurately described the Dynamic Range of light intensities. In essence it is the difference between the darkest to the brightest a camera sensor can capture.

As a the sensors in the cameras dynamic range is not as efficient as our eyes a well-exposed photography is often a compromise of what we see; with the ideal being that the highlights and shadows are not blown out and retain detail.

Most modern cameras have internal metering modes and depending on which metering patterns are used can effect the way an image will be exposed.

  • Centre-weighted – evaluates the entire view but gives more importance towards the centre of the frame
  • Evaluative – breaks the view into smaller zones and averages them across the frame
  • Partial – This takes the metering from a smaller area towards the centre (10-15%) of the frame
  • Spot – similar to partial but even smaller area of the centre 1-5%

There is a really nice explanation on this YouTube video

Metering Modes

With all of these metering modes the camera can set the exposure automatically or manually through the aperture, shutter speed and ISO sensitivity to gain the ‘correct exposure’ (or as at least to the average tone for the camera and conditions). With each mode having more or less usefulness under different lighting conditions. However this raises the question. Do we want ‘correct exposure’ (average tone) for the scene? There maybe times where we want more or less detail dependant on the subject; do I want more detail in the shadows being less concerned with the highlights? or vice versa.

So while the in camera metering modes work we can still override using manual metering or using exposure compensation controls.

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