High Dynamic Range Imaging


High dynamic range imaging (HDRI) is a technique for getting more detail in highlights and shadows.  The principle is to take photographs at different exposures and combine them with software. Consider these three exposures of a restaurant on the coast south of San Francisco.

Three exposures compared

The camera was handheld and the pictures taken in quick sequence. The normal exposure could be improved by lightening the shadows and darkening the highlights in Photoshop. The bright parts of the clouds are so far overexposed, however, that darkening the clouds would could never restore detail.  Once a part of the image is solid white or black there is nothing that can be done to restore detail originally in the scene. There is software deigned to combine two or more exposures to preserve the details. These images were processed with Photomatix, which is available on the web for about $130.  It runs independent of Photoshop and only performs HDRI tasks. The Photomatrix results can be imported into Photoshop for cropping and further adjustments.  Here is the result for the restaurant scene:

HDRI restaurant scene

The image was cropped to get rid of most of the uninteresting road and to enlarge the restaurant. (Barbara's Fish Trap is in Princeton, California. Their fish is very good.) The HDR process produces an interesting image without much fuss.  In the days of film this would require careful exposure using a spot meter followed by elaborate darkroom work. The effort involved in taking three exposures depends upon the camera. Pocket cameras as of this writing usually do not support automatic bracketing. Once enabled, the bracketing feature captures three exposures with three clicks of the shutter. The feature is likely to be included with cameras starting in the $600 to $800 price range, but it also on some less expensive cameras. Instructions for setting up bracketing on a Nikon D80 are given here. Even with the manual, it isn't obvious.

Nothing Can Move

For ordinary scenes with sky and a modest amount of shadows it suffices to take normal exposure and one two stops under. Outdoors with shadows, the three exposure set suffices.  In more extreme lighting wider bracketing is required. A darkened room with a window viewing a bright landscape may require an exposure three or four stops more than the average for the scene. Even fancy cameras do not provide for bracketing that wide, so it has to be done with manual settings.

Here is an another example. Click on the image to enlarge it.

Hawaii HDRI

This is extreme lighting. The picture is taken into bright Hawaiian sun. In the HDRI the shadows remain transparent and sky is blue. (The location is Kawaihae Harbor on the Big Island.)

The Photomatix software includes a Tone Compression module that maps the computed HDR image to a lower range of tones suitable for practical display. Highlights stay unblocked and shadows stay transparent. There are parameters that allow tuning the mapping to suit your taste. I've found that sometimes it is difficult to get enough contrast in output produced by the tone compression software. A solution is to import the Photomatrix  output as a 16-bit tiff image into the Photoshop Elements editor, then adjust the contrast there. It may just be that the Photoshop tools and parameters are more familiar.

The results are enticing, but there are problems. The images have to overlap precisely. The software helps align the photos, so the camera can be handheld. The problem is that nothing within the scene can move while you are taking the sequence of exposures. If people are wlking around or wind is blowing the tree leaves, the method will not work well. Very small differences will blur out acceptably. In the Hawaii photo, the water did not hold still while the three exposures were taken, but the blur in the water is acceptable.

Here is an example where too much has gone wrong:

HDRI with problems

I had to move out of the way between exposures, and did not return the camera to its original position. Also, someone moved in the parking lot. The software could not compensate for the differences and expressed its disgust with black and white artifacts. The moral is not to stand near a restaurant door when taking an HDR sequence.  

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