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Tuesday, October 2, 2012

Beyond ETTR And HDR

32-bit processing in Lightroom

This Article Features Photo Zoom

8) Five-shot exposure-bracketing sequence, plus 32-bit merge in Lightroom

To one degree or another, we've all been underexposing our digital photographs, even if we've been exposing to the right (ETTR). Let me explain. More bits of data produce more tonal values, thus smoother gradations. To calculate the bits of data in a digital file, take 2 (digital files are binary) to the x (the bit depth) power. Higher-bit-depth cameras generate exponentially more tonal values than their lower-bit counterparts. The data in RAW files is distributed logarithmically throughout the dynamic range of a camera's sensor. (The dynamic range of current digital cameras varies between 8 and 12 ƒ-stops, a number that continues to expand.) Twice the light generates twice the voltage and twice the data. Thus, the lightest ƒ-stop contains the most tonal data in a file—half of it. Conversely, the next smaller ƒ-stop delivers half as much light/data as the previous one, so data rendered by these darker ƒ-stops isn't as smooth and noise-free, and is prone to further artifacting and even posterization if it's edited substantially.

Here's the progression:

Imagine a day when every ƒ-stop had as much data as the lightest ƒ-stop. That day is now! Here's how.

Make a series of bracketed exposures where each ƒ-stop in a scene is placed in the far right of the histogram or recorded with half the data in a single digital file. While the ideal number of exposures for such a bracketed sequence is the number of ƒ-stops contained in the scene, fewer exposures may suffice. Exposures one stop apart produce the best results, though exposures one and a half stops apart may produce visual results of nearly equal quality, but avoid using exposures that are two or more stops apart as this tends to produce posterization in smooth areas, like skies.

1-7) Histograms from a seven-shot exposure-bracketing sequence, starting light and darkening in 1-stop increments

Combine all the exposures into a single 32-bit file. Try using either the Merge To HDR Pro feature in Adobe Bridge/Photoshop or Lightroom. If necessary, during the creation of the 32-bit file, you can deal with ghosting artifacts from objects moving between frames:

1. Save or import this 32-bit file into Lightroom 4 (or higher).
2. Apply adjustments with Lightroom's Develop module.
3. Print directly from the 32-bit file or Export derivatives, such as 16-bit TIFFs/PSDs or 8-bit JPEGs. The tonal data in the merged 32-bit file and its derivatives will exceed the data contained in a single exposure dramatically.

Can't you use HDR software to render similar 16-bit derivatives? Certainly. However, if you want to minimize the artifacts created by all HDR tone-mapping software, such as texture/noise accentuation and haloing/vignetting production, try Lightroom. (The current version of Adobe Camera Raw doesn't support 32-bit files, though this may change in a future update.) I find that Lightroom's sliders Highlights (used to "recover" detail in the brightest areas of an image) and Shadows (used to "fill" detail in the darkest areas of an image) have fewer side effects, not only in those areas, but also in the midtones; the Lightroom Clarity slider produces more subtle halo/line effects than other similar software routines, and its noise reduction is more effective and compromises sharpness far less than similar smoothing algorithms.

Current camera manufacturer defaults are outdated. Instead of delivering centerweighted histograms, at the very least they should be measuring the brightest values in the image and weighting histograms as high as possible without clipping (it would be a relatively simple matter to adjust their JPEG-creation routines to deliver normal previews), but ideally they would perform automatic bracketing and 32-bit merging, which would dramatically increase the quality of the files they deliver without hardware advances (though these are welcome, too) and without extra effort on our part. For now, while cameras can perform some exposure bracketing automatically, we have to do all 32-bit merging manually.

It's highly debatable whether the benefits derived from such an intensive postprocessing routine are justified for every exposure you make, but there should be no debate that this routine is useful for images of subjects with extreme dynamic ranges. You literally can shoot straight into the sun and expect to render super-smooth, low-noise detail, even in the darkest shadows. Practice this technique, and you may be surprised to find that you'll enjoy using it even for images with significantly more restrained dynamic ranges. It's a game-changer.

All images are provided by Arctic photographic specialist Ragnar Th. Sigurðsson (www.arctic-images.com), who first alerted John Paul Caponigro to Lightroom 4's 32-bit processing capabilities. Caponigro, author of Adobe Photoshop Master Class and the DVD series R/Evolution, is an internationally renowned fine artist, an authority on digital printing and a respected lecturer and workshop leader. Get his enews Insights with access to hundreds of lessons at www.johnpaulcaponigro.com.