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High Dynamic Range (HDR)

We can be in a normally lit room and look out a window without noticing that the scene outside is considerable brighter than the room we are in. Our eyes usually adjust to the change in brightness seemlessly and most of the time we are not aware of the process. Looking back inside our darker room the eyes once more adjust to the change in brightness. This constant adjustment of the iris in our eyes gives us a perception of a great dynamic range for brightness.

The devices we use to take, view, and print images must record the entire image with one bright/dark adjustment at the instant of capture. If we are inside the imaginary room above and take a picture through the window either the parts of the room that are visible will be too dark or the scene outside will be too bright depending on the camera's exposure settings. In scenes with a high range of luminosity between dark shadows and bright highlights we will frequently find that the shadows are blocked up and the highlights are blown out. That is we will be unable to differentiate between small light levels in the shadows, parts of the shadows will be all black, and similarly the highlights will appear to be all white even though there are birightness variations in these details.

HDR is a technique that lets us combine multiple photo exposures to produce an image that has detail in both the shadows and in the highlights. For example we can take one underexposed image which shows detail in the highlights and almost no detail in the shadows, another image that is overexposed to show detail in the shadows and almost no detail in the highlights, and finally another image that is exposed to show detail in the mid range of brightness. HDR processing involves combining all of these images attempting to use shadow detail from the overexposed image, highlight detail from the underexposed image, and mid range detail from the "normally" exposed image.

We can use HDR in a normal scene outside that has both shadows and bright clouds to bring out detail in the shadows and keep the clouds from blowing out. With a moderate amount of HDR effect the image would not be noticeable as being HDR processed. For other scenes we might choose to turn up the HDR effect to produce an almost surreal image. HDR provides many opportunities to "alter" our images.

There are many sites devoted to HDR on the web, both "how to" and gallery oriented sites. Rumaging around these sites can be time well spent. You will also find a good assortment of HDR software available, some free and some reasonably priced. If you are new to HDR then you have the opportunity to take advantage of some of the free trial offers.

I use Photomatix for almost all of my HDR processing. At this time they have a free trial offer that watermarks the HDR image. The good news is that the trial does not expire.

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