A quick look at HDR

Your eyes can easily adjust to accommodate the lighting conditions of your environment.  If you are in a room with brightly lit areas and deep shadows, the pupils of your eyes will increase or decrease in size to adjust the amount of light so that you can see both extremes in fair detail.  When you photograph a scene such as this you set the aperture (and shutter speed) of the camera to insure the proper exposure for the amount of light present.  If the "dynamic range" is too wide, there may be some areas that are either too dark or too light to see any detail.   The following photographs were taken in my garage (you can click on any of the images to go to full size). Photo 3) was taken with automatic exposure settings, and you can see that there are areas in the photo that are completely overexposed (the garage windows and the light on the clay pot), and areas where the shadows are so dark that detail is lost.  The other images were taken by varying the exposure to allow more or less light to be used.

 1)  Overexposed by 3.3 stops

 2) Overexposed by 1.7 stops

 3) "Proper" exposure

 4) Underexposed by 1.7 stops

5) Underexposed by 3.4 stops







Notice that in the 1) you can see details in the back corner where there is little light, but nearly everything else is "washed out".  In 5) very little detail inside the garage is visible, but you can actually see some details outside the garage through the windows.

HDR techniques allow us to combine two or more of these images so that the overall dynamic range is compressed.  The following two images were created from the above images using HDR software:

 PhotoShop CS5.  (easy, simple, $$$$$)

Hugin.  (confusing, free)







PhotoShop allows you to do some initial adjustments during the HDR creation.  The image using only the defaults was a bit dull and a underexposed using the defaults, so I applied a few adjustments to get it to match the Hugin HDR.  If you enlarge the two HDR images you can see a few differences--for example there is a bit of ghosting in the upper left corner of the Hugin image, but in general they both have a significantly larger dynamic range than the "proper exposure" in 3).

References:
1)  Wiki article on HDR (in depth, but quite good).  http://en.wikipedia.org/wiki/High_dynamic_range_imaging
2)  Creating HDR images with PhotoShop CS5
3)  Creating HDR images with Hugin