HDR is a technique for producing a digital image with High Dynamic Range. Digital cameras unfortunately can only capture a limited range of exposures. So, if you take a picture in a room with light streaming through the windows, the camera often struggles to capture the scene outside. Here's an example I took in my living room - the image of the street outside is largely over exposed and parts of the room inside are underexposed (e.g. the shadow area of the chair in front of us). And even if we could capture such a wide dynamic range, an LCD monitor would struggle to display it!On film, the situation is slightly different. Film has a much wider dynamic range and if I had shot the same image as above on a film camera, if we looked at the negative we would see that the details of the street outside would have been recorded as well as the details in the room (assuming I had exposed it properly!). However, photographic paper (required to make a print)- just like an LCD monitor - is not capable of displaying such a wide dynamic range. This is why traditional photographers end up dodging and burning prints in the darkroom - "burning" refers to exposing the highlights for additional amounts of time under the enlarger to bring out the details. In the example above, the details of the road outside would have been "burnt in" to have produced any detail on the print. Anyway, I have digressed, back to HDR.
HDR is a digital technique for combining images of the same scene that have different exposures in order to capture a wider dynamic range. In other words, you might take three images - one over exposed (to capture the darker areas), one under exposed (to capture the brighter areas) and a normally exposed image. Using HDR software, you can combine these images and effectively "flatten" out or compress the dynamic range. This will add detail to the darker shadows and bring out the detail in the over exposed highlights. It is a pretty handy technique and something where digital photography just beats traditional methods hands down.
Now, often when you see a HDR image, it has often been "tone mapped". Tone mapping is the process of associating a set of approximate tones with an image. Wikipedia covers the topic if you want more details. The resulting images have an almost graphic feel to them. It is possible to adjust the effects in the HDR software, so find something that you like.Here's a test image I produced using Photomatix (you can see it is the trial version by the watermark) with three images - one exposed normally, one over exposed by 1 stop, one under exposed by 1 stop. I have made the effect pretty extreme in this example, but it is possible to turn down effects. You can download the trial software from here on the HDRsoft website. If you want to purchase a license (which allows you to produce images without the watermark), it will set you back around £50.
