HDR ImagingIf 16 bits per channel just isn't enough, Photoshop CS2 supports HDR (High Dynamic Range) imaging, which uses 32-bit floating point data per channel to record an unlimited dynamic range. Photoshop CS2 supports established HDR formats such as Industrial Light and Magic's OpenEXR and the Radiance format used by the open-source Radiance ray-tracing and rendering engine, in addition to Portable Bitmap Format (PBM), Large Document Format (.psb), Photoshop (.psd), and TIFF. The three lattermost formats allow profile embedding, but be aware that color-managing HDR data is an uncertain endeavor at best.Thus far, HDR imaging has largely been confined to the movie industry, and to synthetic imaging produced by ray-tracing applications. Its applicability to "normal" photography is unclear, mainly because we lack any output methods that can handle the dynamic range. The only camera we know that can capture HDR images is SpheronVR's SpheroCamHDR, which is very much aimed at the 3D rendering market. Possibly the most interesting aspect of Photoshop's HDR support is that it allows you to create HDR images from bracketed exposures shot with normal cameras. Merge to HDRYou can create HDR files using Merge to HDR with bracketed exposures in Camera Raw format, or from bracketed exposures shot as JPEG. You can also merge processed files saved in any of the formats Photoshop supports that allow EXIF metadata, but these only work if you haven't made any edits to the file, so using anything other than Camera Raw or camera-generated JPEGs just creates extra work.Merge to HDR uses the EXIF metadata to determine the exposures and blend them accordingly. If you apply edits to Camera Raw files, they're simply ignored by Merge to HDR: If you apply edits to JPEGs (or to any of the other formats), you'll get really nasty results. Shooting for HDRWe find that we get the best results when we bracket by one-third of a stop, though this may be overkill. Bracketing by 1 stop, using enough exposures to cover the entire dynamic range you're trying to capture, often works well. A heavy tripod, mirror lockup, and a static scene all helpMerge to HDR doesn't do well when objects in the scene move. Using Merge to HDRThe process of creating an HDR file is quite simple. You start by shooting a series of bracketed exposures, varying only the shutter speed, that cover the dynamic range you're trying to capture. The easiest way to merge the images to an HDR file is to select them in Bridge, then choose Merge to HDR from the Photoshop submenu (in the Tools menu; see Figure 12-11). Or, if you want to do things the hard way, you can choose Merge to HDR from Photoshop's Automate menu, which produces the Merge to HDR dialog box shown in Figure 12-12. Figure 12-11. Merge to HDR from Bridge[View full size image] Figure 12-12. Merge to HDR from Photoshop[View full size image] Figure 12-13. Merge to HDR dialog box[View full size image] Figure 12-14. Sharpening the HDR image[View full size image] Figure 12-15. Exposure versus Gamma[View full size image] Downsampling HDR ImagesTo do just about anything else with an HDR image, including printing it, you need to downsample to 16-bit/channel or 8-bit/channel mode by choosing one of these options from the Mode submenu (in the Image menu). When you choose 8-bit or 16-bit/channel, the HDR Conversion dialog box appears (see Figure 12-16). It offers four ways of performing the conversion to a lower bit depth. Note that while you can display the Toning Curve and Histogram at the bottom of the dialog box in all four methods, only Local Adaptation actually appears to do anything! Figure 12-16. Downsampling the HDR imageExposure and GammaThe default method, Exposure and Gamma, offers two slider controls. Exposure sets the white point, Gamma sets the midtone. Figure 12-17 shows the HDR image from Figure 12-13 converted to 16 bits per channel using Exposure and Gamma with Exposure set to -1.84 and Gamma set to 1.04. Figure 12-17. Conversion using Exposure and Gamma[View full size image] Highlight CompressionThere are no options for Highlight Compressionit simply is what it is. Figure 12-18 shows the result. Figure 12-18. Conversion using Highlight Compression[View full size image] Equalize HistogramAs with Highlight Compression, there are no options for Equalize Histogram. Figure 12-19 shows the result. Figure 12-19. Conversion using Equalize Histogram[View full size image] Local AdaptationLocal Adaptation offers the most control, but at default settings often produces the least encouraging results. Nevertheless, persistence is rewarded. Figure 12-20 shows Local Adaptation at default settings and the resulting conversion. Figure 12-20. Conversion using Local Adaptation defaults[View full size image] Figure 12-21. Conversion using custom Local Adaptation[View full size image] Figure 12-22. Conversion using custom Local Adaptation and Toning Curve |