Tuesday, June 24, 2008
XDR, Part III
Double exposure gives you two times the image information to use in a final image
How can you tell if a given scene exceeds the dynamic range of your camera (or film)? One, the resulting histogram will exhibit significant clipping in both the shadows and the highlights. Two, exposure adjustment won't improve the situation or allow you to render the image into the dynamic range of your capture device.
Make multiple exposures. Adjust exposure for each, making some dark and some light. At a minimum, get one exposure with excellent highlight detail and a second exposure with excellent shadow detail. Ensure that the histogram rests significantly away from the end of the graph/tonal structure the exposure is optimized for. Ignore clipping in the other end of the graph/tonal structure. The lighter exposures will clip highlights, but reveal deep shadow detail. The darker exposures will clip shadows, but reveal bright highlight detail.
If this isn't the case, you may be able to use a simpler technique, such as double processing or selective adjustment. (See “R/Evolution” in the two past issues of DPP or go to the website to see my previous “R/Evolution” columns on Extended Dynamic Range, www.digitalphotopro.com.) Though you may only need two exposures, a very dark and a very light one, to be on the safe side, make additional exposures. It doesn't matter which end of the tonal scale (dark or light) you start with. Simply work your way up or down from one to the other. Using a tripod, locking down zoom lenses and turning off autofocus will all help you register the two exposures more easily.
Process The Two Files Independently
When processing the two exposures, be sure to maintain a strong tonal structure in the midtones. Favor fully detailed, slightly low-contrast renditions. Guard against dramatically reduced midtone contrast, but above all, avoid posterization. Be careful when applying aggressive amounts of Fill Light or Recovery during RAW conversions or Shadows and Highlights adjustments. How the two tonal structures meet in the middle can be as important as maintaining detail at the ends of the tonal scale.
Combine The Best Of Both: Blend The Two Files Into One
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.
Again, before using complex techniques, see if simpler ones will accomplish the task equally well. Simple contour masks can be useful in images where extreme differences in brightness are separated visually by clearly defined outlines.
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. Watch for posterization. Avoid it. Use it as a guide to help you determine how to split the sliders.