HDR (High Dynamic Range) imaging is a technique to capture scenery with a very large dynamic range, such as sunsets or sunrises. Generally, the range of these scenes doesn't fit on the camera sensor. The technique works as follows: In the field different exposures (in this case, -2 stops, -1 stop, neutral, and +1 stop) are taken with a fixed aperture (only shutter time varies), and later at home these exposures are combined and compressed into a single photo with a computer program. Ideally, the most underexposed image should not contain clipped pixels in white, and the most overexposed image should not contain clipped pixels in black.The combining part results in a 3x32-bit HDR color image. The compressing part, called tone-mapping, downscales this large range to a normal 3x8-bit color image. An important requirement for success is that all the exposures are perfectly aligned. In other words, no camera motion and no motion in the scene itself is allowed within the time-frame in which the series of exposures are taken. In practice, this means working with a tripod and a (wireless) remote control, and of course calm weather.
For this orchid field sunset it did the HDR postprocessing with Photomatix 3.1 using default settings.
There is a lot of debate going on the Internet about whether HDR technique gives realistic results (by the way, what are these?). Plain sensor readouts are certainly not realistic. Playing around with Photomatrix for just an evening gave me at least the feeling that I have something in my hands for the first time in my life to handle sunsets and sunrises shots properly.