HDR imagery demystified
You’re probably starting to here about it all over the place, especially if you spend any time combing through the works at flickr. Every body is doing HDR photography, it seems, and it’s generally becoming the new age way of doing things. So what is it:
(from wikipedia) → In computer graphics and photography, high dynamic range imaging (HDRI) is a set of techniques that allow a far greater dynamic range of exposures (i.e. a large 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 the deepest shadows.
In old school terms, it’s no different than what Ansel Adams used to do in the field and in the dark room, burning and dodging until the sky’s were black and clouds defined and landscapes sharp and perfectly exposed. Or in a little more recent terms, it’s like shooting a landscape on slide film with a polarizing lens to cut the sky down 2 stops, leaving you with well exposed sky AND landscape. What you want is to be able to see the detail in the sky and the detail in the shadows, but to expose that without tricker is nearly impossible.
That’s where HDR comes in. Via computer wizardry, you can take a set of bracketed exposures and marry them together to attain this miracle of perfectly exposed everything. Here is an example of a typical sunset, silhouetted horizon with a brilliantly painted skyline:
But what if you actually wanted to see the foliage in front of you? We can in real life and isn’t that the scene the one the caught your eye in the first place, the REAL LIFE scene? The problem is that most films and sensors have a hard time collecting that 5 to 7 stop range that is presenting before you. Some digital sensors are coming close, especially in RAW mode but they just aren’t going to match what the human eye and brain sees. What you may want is your scene to look like this:
How do we do it? Lets start in the field. I’m not going to give a lecture of exposure right now (though I am sure I will in the future), so I am not going to tell you how to expose each frame in a given landscape but just to tell you that you will want about three bracketed frames; one that is under exposed or exposed for the shadows, one that is exposed somewhere in middle ground (in the above example I exposed for the foliage in the lower right hand corner), and one that is over exposed or exposed for the highlights.
I shouldn’t have to tell you that all three images need to be identically composed so it goes without saying that a tripod is paramount. I should also mention that this is not strictly a digital technique. If you shoot film and scan your photos or negatives, this technique will work for you also. Which brings me to the next stage; get your images on a computer. Once in there, in the old magic number cruncher, open photoshop.
Now you don’t actually need open any file in particular with Photoshop, you just need to go to File → Automate → Merge to HDR.
In the resulting window you’ll select the files you wish to merge, where a minimum of two are required but more often then not, three or more would be used. Now all that’s left to do is click “OK” and watch the magic.
Be warned that your results may vary and the resulting file may need further tweaking with contrast, gamma, saturation etc, but it is a start and takes a large brunt of the work off of you. Just as an experiment I tried to attempt a manual merge of these images myself and in the hands of a proper graphic artist, the task would not be impossible, but to accomplish the same results in the 30 seconds that Photoshop can spit these things out would be.