No doubt you have taken a photo where either the foreground had the correct exposure while the background was blown out or too dark or the other way around. So have I and if you have a digital camera there is a lot you can do, especially when you're shooting in RAW. From Wikipedia: High dynamic range imaging (HDRI) is a set of techniques that allows a greater dynamic range of exposures (the range of values between light and dark areas) than normal digital imaging techniques. The intention of HDRI is to accurately represent the wide range of intensity levels found in real scenes ranging from direct sunlight to shadows.
Preferably you would use the autobracketing function of your camera to take several exposures; typically a camera would take a burst of three frames with 0, +1 and -1 stop (now called EV) difference compared to the normal exposure. If you shoot in RAW you can fake this at the cost of some noise in the image.
For the image above, I had only one exposure and when I had the musicians exposed correctly, London in the background was completely blown out and had lost all its detail. Since I had shot it in RAW, I could manipulate the exposure in Adobe's Camera Raw to create three TIFF files with 0, +1 and -1 EV. I then used that in the free tool Picturenaut to create the image above which is already a great improvement over the original image I had made. However, Picturenaut has a lot more options I haven't had time to look at, so no doubt, I will be working on the above image while I discover more and more about HDRI.
Note that unless you have Photoshop CS2 or above, Photoshop cannot handle the HDR TIFF files. However, Gimp can.
Camera: Pentax *ist DS, lens: Sigma 18-50mm F2.8 EX DC MACRO + Picturenaut
