Digital cameras have a drawback compared to their analog ancestors. That is, they are not as good at capturing the full dynamic range in an image (although, the newest digital cameras are becoming much better at this). Depending on the subject being photographed and this would usually be outdoors in bright sunshine, part of the resulting image might show an exposure that is overblown and, at the same time, part of that same image is underexposed. To overcome this problem, HDR photography involves taking multiple exposures of the same scene. Often, three exposures are sufficient. One is overexposed, one is underexposed and one is at normal exposure. Many cameras have this exposure compensation feature built in. Once set, the camera will automatically take 3, 5 or even 7 exposures at different values to capture bright and dark areas in the scene at the correct exposure value. Loading these images in an HDR software package will result in one image with the proper exposure for all areas of the scene. I have found that this HDR process is particularly effective when photographing dimly lit subjects. Photographing indoors without a flash or outdoors at night are usually good times to take multiple exposures.
Within that same HDR software package is a process called tone mapping. This process maps different tonal values into each pixel. Instead of having tonal values from pure white to pure black, the tone mapping process reduces that full dynamic range to a more limited dynamic range thus eliminating the pure whites and the pure blacks. It is usually possible to adjust the amount of tone mapping in the software.
At this point, it should also be noted that one can choose to render a single image to a tone mapping process. A highly tone mapped image will likely begin to look more like a painting than a photograph. Some photographers hate it and others not so. I suppose I am one that kind of likes the effects of that kind of tone mapping.
For example, this is a picture of the inside of the Mariners' Cathedral in Detroit MI. This is an HDR image. Three pictures of this subject, overexposed, underexposed and normal, were used to create this final HDR image.
With the following image, only one picture was taken. This is a picture of the Elmhurst Inn in Ingersoll, ON. This image was simply tone mapped (using open-source Luminance HDR). As you can see, the tone mapping process in this image produced a result where the photograph begins to resemble something like a Thomas Kinkade painting.
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