HDR Tone Mapping vs. Exposure Fusion (What’s the Difference and Does it Matter?)
One of the oldest challenges in photography is to capture important details in both highlight and shadow areas. Unfortunately, film or digital cameras can only record a finite range of brightness. The introduction of High Dynamic Range (HDR) software in recent years has become a popular way of dealing with this challenge. In addition, options within most programs also offer stylized outcomes. These typically have more saturated colors, higher levels of contrast and extraordinary renderings of details in shadow and highlight areas. Such treatments, with names like “painterly,” “grunge,” “surreal” and “artistic,” are often collectively referred to as having “the HDR look.”
The HDR process begins by taking a range of different “source “exposures so that important shadow and/or highlight are captured with details. All this data, however, represents a brightness range too large to display on a monitor or be used by a printer. HDR software then processes this HDR data into a low dynamic range (LDR) file suitable for display and printing. The common mechanism used is the tone mapping process, which is also the main player in producing the various HDR looks.
I have seen many obviously tone mapped images that are quite striking and highly original in the interpretation of different subjects. But very often I will want to render a wide dynamic range scene with realistic colors and contrast levels as well as having important details survive in shadow and highlight areas. As a result, I will tend to avoid most tone mapping options and use software offering the Exposure Fusion (EF) method.
The term, “Exposure Fusion” was used by Tom Mertens and others in a 2007 scientific paper to describe a method of building a final image pixel by pixel from the source exposures. (Mertens et al 2007, Exposure Fusion: A Simple and Practical Alternative to High Dynamic Range Photography). This pixel selection is based on the most appropriate exposure values as determined by the software’s algorithms. ER processing does not require a tone mapping stage so the result is a wide dynamic range without any hint of the HDR look. Admittedly, many photographers will find such “straight” results as rather tame especially with all the interest today in more intense HDR tone mapped images. Nevertheless, I find EF processed images (or “toned down” tone mapped images) are a better fit for my particular photographic style.
Source Images: Three source images were taken of a construction site using the equivalent of two f/stops apart by changing shutter speeds on a tripod mounted camera. This exposure range captured important details in highlight and shadow areas. All of the following software processed images were then done using default settings. Note: These results are not meant as an absolute evaluation of the specific software programs but rather to present some idea of the range of results. The reader is encouraged to test software with demo versions to see how much the default results can be altered.
Results: (1) This image was produced with Bracketeer, a photo utility that does not use tone mapping but rather infusion methods. (http://www.pangeasoft.net/pano/index.html). I particularly like the subtle rendering of shadow details and the way it preserves the colors from the source images. (2) Slightly more saturation and contrast was produced with the “Exposure Fusion” choice in Photomatix Pro (www.hdrsoft.com). This software is very popular for its tone mapping options. (3) The HDR “Realistic” tone mapping choice in HDR Efex Pro software gave results very close to Brackeeter but with more saturation and some color shift compared to the source images (www.niksoftware.com). (4) Stronger levels of saturation and contrast are more prevalent in the “Default” HDR tone mapping setting for Photomatix while the remaining two images, (5) “Artistic Structurized” from HDR Efex Pro and, (6) “Painterly 2” from Photomatix illustrate the most dramatic tone mapping effects.
A search of the web using the term, “Exposure Fusion,” and “HDR Software” will turn up other programs that you can evaluate based on your needs and preferences. A more complete treatment of the techniques for capturing scenes with wide dynamic ranges can be found in Chapter 3, “High Dynamic Range and Exposure Fusion Photography,” in the book, Advanced Imaging, by Joseph Meehan.