The Theory Behind HDR

Software pointers
With respect to software, there are many different options ranging from free utilities through to Photoshop CS5. The market leader is Photomatix Pro by HDRsoft. Turn to page 96 for reviews of three HDR softwares. Whichever you use read the manual first because it will tell you how many pictures you need, what format they should be in and how aligned they need to be in order for the software to work.

The software will do different things and it is helpful to have an idea of what the techniques do to the data when trying to choose which to use. There are two main branches of HDR processing: tone mapping and exposure fusion, and they differ fundamentally in what they do. Exposure fusion takes the data and creates a weighted average for each pixel so the shadows will have their data taken mostly from the over exposed frame and the highlights will have their data taken mostly from the underexposed frame. The result from this processing is a very natural looking but flat contrast. It does not substantially alter the rendering of the detail and the file it creates is immediately useable, since the data is compressed into a range that printers and screens can display.

Tone mapping refers to the way that the software displays an HDR image. The actual file is an HDR file, with each level of light recorded correctly, but you would not be able to see this on a print or on screen. The tone mapping process looks at the data in the file and renders it in a way that can be displayed, which then allows you to save the version for use. The file retains all the data and can be saved for alternative tone mapping in different versions. Tone mapping offers many additional options allowing you to alter local or global contrast within the scene and is the technique that produces the more stylised HDR images that have become so popular.

The elements that can be manipulated in exposure fusion relate mainly to the way that the averaging is weighted: in essence what the controls do is to adjust the curve of the image contrast.

In tone mapping the software interprets the image according to the distribution of tone in the picture. In areas where there is relatively low contrast it can boost the local contrast to emphasize detail. This means that taken as a whole the relative density of different areas in the image are somewhat elastic, so it is possible for darker areas to end up lighter than parts of the picture that started out lighter in the first place. This is, for me is often too much, it goes beyond what can reasonably be called photography and becomes digital art. It is essentially the work of the software rather than the skilled use of a tool.

In conclusion, HDR photography is very clever: used well it can produce superb results with amazing detail in highlights and shadows but still looking very natural. It is not an end in itself though, as first you need a great image with bracketed exposures that you can enhance. If your source picture is dull then the result will almost certainly be a dull HDR.

Taken from the February 2011 issue of Advanced Photographer magazine