Friday, October 05, 2007

High Dynamic Range Images

I've just spent some time playing with Photomatix an interesting little tool for creating High Dynamic Range images.

If you have a digital camera, you probably know that they have a very limited dynamic range. Imagine you are taking a picture of a person sitting under a sun umbrella. If you set the exposure for the person, the surroundings will be over exposed. Set exposure for the surroundings, and the person will be underexposed. No matter how you adjust the settings, the camera doesn't have enough dynamic range to capture detail across the scene.

HDR photography is kind of a trick. Put the camera on a tripod and take three (or more) photographs; one correctly exposed, one under exposed and one over exposed. Then merge the three images to create a single image with a higher dynamic range.

That's the basic theory.

Well, pretty simple really; get a camera and a tripod. Many digital cameras have an auto exposure bracket function; turn the feature on by setting how many stops you want to over and under expose the image, then push the shutter and it takes three images in succession.

Then you transfer the images to a computer (Mac or Windows) and load them into the software. The merge process has only a few options and is basically automatic, but it creates a preview image that won't look right, You then have to "Tone Map" it, and then you have options to adjust settings and the output format.

The Results
The short answer is: mixed, but promising.

The Details
For the test, I put the camera on a tripod and literally walked around snapping some scenes I thought were interesting. I used a Canon PowerShot S-5 set to +-2 in Program mode, and didn't try adjusting exposure options.

The software does an interesting job. Of the seven or so tests I tried, two were definitely an improvement, but the rest were a mixed bag. I still have to play with the software options; for the moment I found it easier to bring the results into Photoshop and adjust the Contrast to create something I found pleasing. But I still had problems with the contrast across some images looking completely wrong, and an 'artifical' color feeling in others. There was noticeable color fringing in two, and a patterning in one set that I can't figure out.

And a big problem is that you really have to use a tripod, and don't want to have things moving in the scene.

I was using the trial version, and I'm still not sure if I want to shell out the $99 for the non-watermarked version. But I'm not giving up just yet. I want to do a few more experiments because for those two pictures that did work, it was quite impressive.

The example below shows a final image, and the standard exposure image (insert.) Note how much of the detail in the bench is blown out, and how dark the trees in the bakground are in the single exposure image.