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Wednesday, March 23, 2011

What is HDR and why is it important?

   Imagine you go to the park on sunny day with a few puffy clouds in the sky.
 The grass is a rich green and the sky a deep blue with brilliant white clouds.  Sitting in the shade of a tree you see someone reading a magazine.

  It's a typical scene and you can see all aspects of it at once.  You take out your digital camera to make a record of this and you end up frustrated.  You can take a picture of the green grass but then the sky is a washed out blue and it's hard to see who is sitting in the shade of the tree.  You can photograph the sky but then the grass turns out dark and you can't even see the person in the shade of the tree.  Or you can photograph the person but when the green grass looks mostly white and the sky itself is all white like a bulb.

  The problem is that digital cameras can take pictures over a range of light intensities that's far less than our eyes and brain can perceive.  So you have to chose which of the three parts of the beautiful park scene you want to record and that's just frustrating.

 This is where High Dynamic Range photography comes in.   With HDR you take a sequence of pictures with the exposure set to capture three different ranges of light intensities.  You keep the camera still - you just change the exposure settings (generally just the shutter speed since aperture changes can change the focus).    Then you use special software to combine the three pictures into one, using the properly exposed parts from each picture.

 In our park example you might take one picture at the normal exposure determined by the camera to be suitable for this scene.  Then you would overexpose the next shot by two F stops and underexpose the next shot by two F stops.

 The preceding picture of the view of San Francisco from the beach was done that way.  That allowed the grass near to camera to be exposed for properly as well as the sky which was much brighter than the ground, even on this cloudy day.

  In subsequent posts we'll got through some examples and discuss the software.


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