Monday, April 28, 2008
XDR, Part II
Extend the dynamic range through this processing technique
Reproducing the full range of tones or brightness values seen by the human eye is one of the most fundamental challenges in photography. Capture, display and print technologies are all limited when compared to the ability of the human eye to see a wide dynamic range. Recent advances in technology enable you to exceed these limitations.
Techniques for extending dynamic range span the gamut from simple to complex. Before resorting to complex solutions, try simpler ones first. Apply the principle of Occam's Razor. (See “Introduction To XDR” in the previous issue of Digital Photo Pro for more on this.)
If you need to make an image of a relatively high-contrast scene that challenges but doesn't exceed the dynamic range of your camera (or film), consider double-processing. Make two derivative files from one original—one dark and one light. Layer the two together. And blend the best information from each. This practice can substantially improve the quality of the information in your file. (It's not for every file. If you can achieve ideal results with one layer, do. Again, apply the principle of Occam's Razor.)
Get The Best Information—Make Two Files
If the dynamic range of an image challenges but doesn't exceed the dynamic range of your camera (or film), you can process a single RAW file or scan a single piece of film twice. Make one light version for ideal shadow information; don't worry about clipping highlights. Next, make one dark version for ideal highlight information; don't worry about clipping shadows. It doesn't matter which version you start with, light or dark, as long as you end up with two separate files, each with ideal information at opposite ends of the tonal scale.
In addition to good information (detail and expanded tonal structure) in shadows and highlights, be sure to maintain good information in midtones. Avoid posterization and dramatically reduced midtone contrast. Be careful when applying aggressive amounts of Fill Light or Recovery during RAW conversions or Shadows and Highlights adjustments. Because you're processing two files with the intent of layering them together, you may not need to be as aggressive with these tools as you normally would be. How the two tonal structures meet in the middle is almost as important as maintaining detail at the ends of the tonal scale.
This principle applies to subsequent adjustments of the separate files, as well. In general, favor full detail in slightly low-contrast renditions that you then can adjust further (with adjustment layers). Doing this will help you get better blends.
You can use layers to blend the best information from both light and dark files. Drag the Background Layer from one document to the other. (Title them appropriately.) Generally, it doesn't matter which layer you place on top. What matters is how you blend the two.
You can use the Blend If sliders found in the Layer Style menu to restrict what values of a layer are visible. Double-click on the layer to activate the Layer Style menu. Then use the sliders in This Layer to remove the effect on the Background layer. For smoother transitions, feather the effect by holding the Option key and splitting the sliders apart. Avoid posterization. Use it as a guide to help you determine how to split the sliders.
If posterization occurs, you can more precisely blend a specific range of the tonal scale by adding a luminance or contrast mask. Go to the Channels palette and Command/Control-click on the RGB channel. This will load a selection of the highlights. If you want to create a selection of the shadows, go to the Select menu and choose Inverse. Add a layer mask by clicking on the mask icon at the bottom of the layers palette. The selection will automatically become a mask, restricting the adjustment to the specified areas. To refine transitions, you can modify the brightness and contrast of the mask by applying Curves to it (Image > Adjust > Curves). You can further refine these masks by locally adjusting or painting on them. (For more on this, see Resources under the (R)evolution tab at digitalphotopro.com.)
If posterization still persists, you may need to triple-process the file.
While blending the two images, apply the same principles and be mindful of the same concerns during acquisition. Avoid posterization and dramatically reduced midtone contrast. Aim for full detail with slightly low contrast but good midtone separation and naturalistic blends between the two versions. This will provide an optimum base image that you can enhance without encountering undesirable artifacts.
Once the best blend is achieved, further enhance the blended image (with adjustment layers that adjust both image layers simultaneously). Don't flatten. Keep your layer stack intact to preserve your ability to make future improvements. Use a flexible workflow.
But Wait, There's Still More…
There are limits to how far you can go with this technique. You can't exceed the dynamic range of your camera (or film). During exposure, if a histogram is significantly clipped in both the shadows and the highlights, take this same principle to the next level. Expose once for the shadows, once more for the highlights, then layer the two exposures together. Using this method, you can exceed the dynamic range of your camera (or film).
Part III of using XDR will be continued in the next issue of DPP.
A member of the Photoshop Hall of Fame, internationally renowned fine-art photographer John Paul Caponigro is the author of Adobe Photoshop Master Class. He teaches an array of workshops in his private studio. Get more than 50 free PDFs and test files, including techniques related to this article, at www.johnpaulcaponigro.com.