New Photoshop Toy

The New York Times tipped me off to a technique in photoshop that lets you "cheat" in capturing the dynamic range of a scene in a photograph. The technique is mostly automated in the full professional version of photoshop CS2 and is called HDR, which stands for high dynamic range.

In a nutshell, dynamic range is the difference between the light and dark areas of a scene. The human eye has a pretty impressive dynamic range in that we can be in bright sunlight and make out details in the sky while simultaneously being able to see things in the shadows reasonably well. Camera film has something on the order of, if I remember correctly, only about a 10th the dynamic range of the human eye and slide film fares worse in this department than negative film (what most of us shoot to get prints back from the processor). This should be obvious to anyone who has ever taken a photo of someone against a bright sky, only to find that either the person's face is a silhouette against a nicely exposed sky or the face is nicely exposed against a washed out (white) sky. Film just can't cope with the contrast. Digital cameras have even less dynamic range which is quite obvious if you've ever used film, especially good quality negative film that is lower in contrast than the typical drug-store variety.

So HDR offers a partial solution. It takes three or more of your photographs of the same image, exposed at different levels, and combines them to simulate a higher dynamic range, approximating human vision. One picture "underexposed" (or rather properly exposed for the highlights), one "overexposed" (properly exposed for shadows), and one "properly exposed" (captures midtones). It is, at least in my trial run, a fairly painless (aside from the $650.00 for photoshop!) procedure that gave me the following image with little fuss:

Of course, I read the manual after taking the pictures. I did not expose the images properly. The sky in the upper right (where the sun had just set) is still overexposed and the shadows in the lower left and trees along the Palisades are still underexposed. Still a reasonable image, exposure-wise, compared to the originals. Also pretty realistic looking compared to some of the images in the Times article. Originals:

Overexposed

"Properly" exposed

Underexposed

One of the obvious limitations is that the scene in the three pictures must be virtually identical in content - no movement. A tripod is a virtual necessity. So it's great for still life's and landscapes, but not much use for sports or kids running around playing in the park. If your camera has auto-exposure bracketing it may be helpful but it may not give you enough range by itself - still a nice way to fire off three quick shots in succession without fiddling with knobs or buttons between shots and risking that the camera will be bumped or the scene will change. So get your school to purchase a copy of photoshop (or try one of the less expensive software programs specialized for HDR rendering mentioned in the Times article) and have some fun with it. Check out some of the fun stuff by Kris Kros, also mentioned in the Times article.