Monday, August 21, 2006

hdr photography

bracket exposures on the left, HDR image on rightPaul’s note: My photographs of spectacular scenery most always disappoint, especially in challenging light conditions. In his first guest contribution to (at least) one cool thing, Don, of Banana Joe’s Pineapple Frosty fame and an expert in many domains, explains why this happens and details how HDR techniques can solve the problem and produce amazing results.

Those of us who have gone shopping for a digital camera know it is all too easy to fall into the ever-escalating arms race for more higher and higher resolution. For instance, Canon's EOS Mark II now sports a massive 16.7 megapixels per image. For most of us not in the business of printing out billboards or posting images to the nearest JumboTron, this sort of pixel density is not often necessary.

A less commonly recognized aspect of digital photography is dynamic range. We have all run up against the inherent dynamic range limitations of our cameras numerous times. It happens when you encounter a large variation in light intensity within the same scene. On a sunny afternoon you compose a picture of your friends in the sun with an interesting landmark off to the side in the shade. You press the button on your trusty camera and *click* you see your friends but the landmark has disappeared into a featureless shadow.

What happened? Once again we learn that our eyes are simply amazing optical instruments. They have a dynamic range far superior to any consumer digital camera. Your eye-brain combination can perceive details under lighting conditions that vary by a factor of over 10,000 fold within the same scene. In other words, the have a dynamic range of over 10,000. A typical digital camera, however, has a dynamic range on the order of about 500. This means simply that your eye can see many things when you compose your shot that your camera isn't going to pick up if there are large variations in lighting.

But what to do? While camera manufacturers perfect sensor chips with improved dyanmic range, amateur photographers are increasingly turning to software based processing solutions to increase the effective dynamic range of our cameras. There are a variety of software packages available that allow you to merge multiple bracketed exposures of the same image together into a single high dynamic range (HDR) image.

A simple way to generate an HDR image is to take three shots, one with a relatively long exposure time to capture details in the shadows, one middle exposure (the one your camera would automatically select) and one short exposure to get the brightest portions of the image. Many cameras including my trusty Canon Powershot include an auto-exposure bracketing feature that will automatically fire three shots with the requisite exposure settings. With the default setting, the three bracketed images span an extra 4 f-stops worth of dynamic range.

I've been using the software from Photomatix ($99) and have been happy with the results. Photoshop's CS version is also very popular for creating HDR images. One of the most successful HDR images I've made so far came from one of Elaine and my hikes in the Columbia River Gorge. I wanted to photograph a brightly sunlit waterfall that fell before a massive rock overhang. The angle of the sun kept the overhang and the trail below in deep shadow while the light reflecting off the waterfall was very intense.

Longer exposure times caused the water to be overexposed and "blown out" while shorter shutter speeds caused the details in the shadowed area to disappear. (photos above on left) A steady hand combined with Photomatix's auto-align feature allowed me to produce a composite image that I felt more accurately captured the scene as I saw it. (large center image) Remember though, because you are compositing multiple images this method works best for subjects that are not moving!

I prefer to use HDR methods to produce images that I feel are closer to what my eye saw when I composed the scene, but many people also use HDR software methods to make striking surrealist images. Whatever your taste in photography, HDR processing can be a very powerful tool.

Bonus link: Top-rated HDR photographs on flickr


At 9:49 PM, Blogger warrenjp said...

This is totally cool! Thanks, Don!

At 4:05 PM, Blogger paul said...

an amazing shot of Tokyo at night:


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