Scenes with very high dynamic range (containing very bright and very dark sections) are difficult to capture in a single shot without either blowing out the highlights or underexposing the shadows. Although common sensors in current DSLRs have 7 to 8 stops of dynamic range, many natural scenes would normally have a much greater dynamic range, too large to be captured accurately, even though they would look perfectly normal to our eyes. This is because each photoreceptor in the retina can adjust its gain independently (or almost independently) of each other, while an ISO setting in the camera applies equally to all pixels in the sensor. HDR imaging is a way to extend the dynamic range of a capture by taking sequential exposures of the same scene with bracketed settings. High-end DSLR cameras can do this automatically after dialing in the desired bracketing sequence and interval. Typically one shot is taken at the “correct” exposure indicated by the metring system, while additional shots are taken above (say +1, +2 or +3 EV) and below (say -1, -2 or -3 EV). The trick is to get all highlights and shadows correctly exposed in at least one of the exposures in the series. The images in the series are then combined to generate an HDR image using specialized software. Later editions of Adobe Photoshop can do this. Dedicated HDR software, such as Photomatix, is however easier to handle and yields very nice results.
The three photographs below -taken during a recent visit to Pompei- were shot at 1/125, 1/500 and 1/2000 sec, respectively, on the EOS 40D with EF-S 10-22mm lens at 15mm, ISO400, f/11.0 (always apperture priority for HDR). The corresponding histograms are shown under each image:
Detail in the shadows is totally lost in the first image, but all highlights are inside the histogram and the sky exposed correctly. The opposite aplies to the far right image: shadows are now resolved but highlights are completely blown out. The middle image is the “normal” exposure, what one would have been left with from this scene had one not decided to take bracketed images.
Below is the HDR image produced by Photomatix from the three photographs:
A well exposed, nice image, with detail in the highlights and the shadows, just like my eyes saw it! Incidentally, the marked diagonals add to enhance the composition here. Very pleased with this one!