Monday, March 3, 2008
Introduction To XDR
Extending the dynamic range in your images is possible with sound technique and a little software magic
If you shoot RAW files, you not only have more data (16-, 14- or 12-bit versus 8-bit), but also have the option of processing it yourself with a RAW converter rather than having the camera process it for you. This helps ensure data isn't lost during development.
The RAW files you shoot have more data in the highlights than is indicated by the camera's histogram. Typically, there are many more bits of data in the highlights than the shadows. Assuming your photo has a six-stop dynamic range, the progression looks like this from dark to light: 64, 128, 256, 512, 1024, 2048. Consequently, data in the shadows tends to be noisier and of lower quality. For this reason, when shooting RAW files, favor overexposure rather than underexposure. Don't clip unless you have to, but do keep in mind that some clipping in the highlights of a JPEG preview may not indicate loss of detail in a RAW file. You'll be amazed at how much highlight detail you can recover.
|1) Curve to reduce contrast||2) Before|
|3) After; Curves set to Normal Blend Mode||4) After; Curves set to Luminosity Blend Mode|
Once you've taken steps to ensure you have the best exposure possible, you can take additional steps to render images with high contrast in a way that's more pleasing to the eye. The dynamic adaptation that the human eye automatically performs when it encounters high levels of contrast, opening up and closing down over many moments, can be simulated in a photograph made in a single moment.
Curves, the most precise tool for modifying brightness and contrast, allows you to target and adjust shadows and highlights independently of one another. Use it to reduce contrast and render detail in very bright highlights and very dark shadows. (Remember, if there's no detail due to clipping, no tool can restore it.)
Using the Curves interface, hold the Command/Control key and click on the area(s) of concern and make an appropriate adjustment. During this process, it's likely that you'll also have to adjust values at the other end of the tonal scale and midtones to generate the best results. (Try building a precise curve before resorting to making selections or masks for areas of concern. Often, this simpler solution will deliver excellent results.)
Use Curves during RAW conversion; it's more precise than Contrast. If you're dealing with scanned film originals from scanned film, make these adjustments as adjustment layers in Adobe Photoshop. Guard against reductions in saturation that typically accompany reductions in contrast. To compensate, boost saturation during conversion or make Curves adjustment layers set to a blend mode of Luminosity.
Page 2 of 3