Have you ever tried to take a picture and the contrast of the scene was to much; or in other words, was there was a portion of the image that was either way to bright or way to dark, so much so it ruined the picture? This is because what your eye can see and what the camera sensor sees aren’t quite the same. In photography this is referred to as dynamic range. The human eye has a wide dynamic range so you can perceive more definition between bright sand dark subjects as opposed to a camera sensor. To compensate for this deficiency in dynamic range High Dynamic Range (HDR) photography has been developed.
HDR is a relatively simple concept. I’m going to make up some numbers here to help explain. Say the human eye has a dynamic range of 10 (units aren’t important for the example) and your camera has a dynamic range of 4. So if you take a picture of the same scene three times, 4+4+4>10, you would be able to not only replicate but perhaps exceed what your eye can perceive depending on how much over lap there is between the pictures. That is exactly what HDR photography is, you take multiple images and combine them to form one image. Typically this is done in odd increments such as 3, 5, 7, 9 and so on but there is no rule besides whatever it take to capture the image the way you want to.
What is challenging about take HDR images is when there is motion in the scene. It becomes a lot harder to match up you pictures if people or things moved around between frames. This is why most HDR is shot on a tripod and of still life or landscapes.
Typically when I’m taking an HDR shot I’ll do a picture that is underexposed or darker than usual, then I’ll take a normal exposure, and lastly I’ll take an overexposed image which is a brighter picture than usual. I normally stick with three images unless the range of the scene is extreme then I’ll do five.
Once you have your pictures the next step is to combine the images. This is done via software and there are a number of resources out there to learn how to do this. One of the leaders in the technique, Trey Ratclif, runs a website called Stuck in Customs that offers an exceptional tutorial on HDR. I highly recommend you try it out. I’ve tried a number of software but I always find myself coming back to Photomatix. You can download it for free and try it out, the paid version doesn’t cost much (in comparison to some software i.e. Photoshop) last I checked it was $99.