January 8, 2009
Each series of bracketed source images needs to be combined into a single 32-bit High Dynamic Range Image. This 32-bit image can contain much more information than any 8- or 16-bit file type.
The real-world luminance values captured from each shot in the bracket sequence are preserved in the merged 32-bit file. It's a little confusing at first, but the overexposed shots are used to capture the darkest shadowtones, and the underexposed shots are used to capture the brightest highlights. And in a 32-bit file, there's enough headroom that nothing is clipped to pure blacks or whites.
Adobe Photoshop CS4 and a number of independent programs such as Photomatix Pro, Dynamic Photo HDR, and FDRTools Advanced are able to combine your bracketed images into a single 32-bit file. Each does the process with a different interface, but the end result is always a 32-bit High Dynamic Range file.
Adobe Bridge CS4, which is part of Adobe Photoshop CS4, has a cool new feature under the Stacks menu called Auto-stack Panorama/HDR that analyzes your folder of images and automatically stacks similar shots for panoramic stitching or HDR merging.
Whether you select an Auto-Stack or simply highlight each bracketed image, the next step is the same. Tools>Photoshop>Merge to HDR will combine your multiple shots into a single High Dynamic Range image.
As you can see from these two screen shots, the 32-bit file contains much more visual information than each individual source image. There's no black or white clip—we are just previewing a different exposure value slice of the same file in these two screen shots.
*Practical HDRI includes screenshots and tutorials for Adobe Photoshop CS3, but there is almost no change in functions or operations between versions CS3 and CS4.