Compressed Dynamic Range Saturday, March 31st, 2007
There is an in-depth Photoshop tutorial in this article, so make sure to read the whole thing!
There has been a good deal of activity and interest lately in the area of “high dynamic range” or HDR imagery. Many of these “HDR” images are characterized in part by a surreal or otherworldly feeling accompanied by an ethereal glow surrounding the subject. I think the term “HDR” was ill-conceived in this context. There is such a thing as a high dynamic range image, but you wouldn’t be able to view it because it contains more tonal data than you can perceive or that a computer monitor could display. A true HDR image would be processed into viewable images by compressing its range; essentially by running it through Photoshop’s “levels” or “curves” (but not in Photoshop, because Photoshop probably couldn’t open it).
The HDR images you see on Flickr and PhotoSIG are actually compressed dynamic range because they portray scenes that had too much tonal range to be captured in a single frame; the range of tones from highlight to shadow are compressed into the range of a normal image. This is usually achieved by making two or more frames at different exposure settings and combining them using Photoshop or one of a number of standalone and plug-in softwares for that purpose (such as Photomatix, PanoTools, etc.)
It’s easier to understand (for me, at least) using pictures, so here is an example. The two photographs below were taken of the same scene, a scene with too much range for my camera to see full highlight and shadow detail at the same time. One of the photos is exposed to capture the highlight data completely and the other is exposed to capture the shadow data completely.
Below the photos you can see their histograms (I just snagged these from Photoshop’s histogram palette). Notice how photo #1’s histogram is biased toward shadow data and photo #2’s is biased toward highlight data. You can see from the pictures themselves that #1 has a much cooler looking sky, while #2 has the detail you would probably want in the subject (in this case, a humble charcoal grill). (more…)
