Tuesday, September 30, 2008
XDR, Part V
HDR with Photoshop
|4) Final image|
HDR imaging represents a radical departure from traditional forms of photography. HDR (high dynamic range) /images hold more luminosity than conventional cameras can record and potentially more than the human eye can see (at any one given time). Containing the full luminance values of a scene, not bound by an output referring standard, exposure is set after capture when you render it into an LDR image. You can “reexpose” this HDR scene digitally as often as you like by taking a new snapshot of it. The extra data gives you astonishing fidelity and flexibility. HDR imaging is so fundamentally different that core concepts in photography need to be reconsidered. HDR /images need to be made in different ways than LDR /images.
Preparing Exposures For HDR Merges
Few true HDR capture devices are available or affordable, but you can create HDR merged /images from multiple bracketed exposures with available cameras. A little preparation makes it possible.
To make a 32-bit merged HDR file in Photoshop, you need multiple bracketed exposures. (There’s little benefit to merging one file processed in different ways, as this won’t expand dynamic range.) You can get away with as few as two exposures, though typically three to six are used. HDR merges don’t accommodate moving subjects well; they produce strange artifacts. To help ensure registration, exposures made on a tripod are favored. Many cameras offer the ability to auto-bracket burst exposures; sometimes these fast brackets can be successfully merged into HDR files. Bracketed exposure values are often best when set one or more ƒ-stops apart. Make exposures set to aperture priority so depth of field doesn’t change between shots.
|1) Four exposures of the image |
Merge To HDR
Many software packages are available for creating HDR merges. Adobe Photoshop is the most commonly used. (For an excellent resource on HDR, with reviews on a variety of the best available HDR software packages, see Christian Bloch’s The HDRI Handbook.) Photoshop can simulate HDR capture by merging two or more different exposures of a scene. The Merge to HDR feature will first align the exposures; Photoshop’s automatic alignment features are impressive. Next, Merge to HDR will blend the different /images into a 32-bit HDR image; Photoshop’s functionality is limited in 32-bit mode, but you can convert 32-bit HDR /images to 16-bit or even 8-bit LDR versions and edit them normally at any time. Finally, Photoshop’s conversion dialog offers a variety of methods for converting merged exposures, rendering HDR /images into ideal, usable LDR /images.
|2) HDR Merge dialog box |
|3) Alignment of images|
Generally, the automatic alignment features provided during Merge to HDR do a very good job, particularly if you took some care during exposure. When they fail, you can manually align /images before conversion to 32-bit. This is tedious and can take substantial computing power, but it’s possible.
Stack separate /images as layers in a single document. Turn the top layer to the Difference blend mode; the image should appear very dark. Move and/or distort (Edit > Free Transform) until colored halos are reduced as much as possible. Once transformed, turn the layer’s blend mode back to Normal and turn it off. Repeat this process with the other layers until they’re all aligned. Then turn on all the layers and create a 32-bit HDR merged file.