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Friday, April 1, 2005

Getting The Most From RAW


A valuable tool for the digital photographer, RAW must be treated with respect to maximize its benefits


The key: Shoot it right in the first place. If you have any doubts, try another exposure. There's an idea that bracketing isn't ever needed with RAW, which could really shortchange you in a difficult lighting situation. Let's look at a few problems that come from poor photographic technique when shooting RAW.

Underexposure. The worst problem, if you underexpose a RAW file so that the tonal information is mostly in the dark areas (the left side of the histogram), you're not using the sensor most efficiently. When you brighten those dark areas, you don't have the best tonal or color information to begin with, plus you're brightening the most common place to find noise—dark areas. Even the best digital cameras will start showing annoying noise when an image has been underexposed. Remember, you always can darken a lighter exposure to keep noise out and colors in.

This is why you can't simply rely on your camera's LCD to tell if your exposure is correct. With a lot of practice comparing LCD images to what opens on your computer, you can judge images on the LCD, but a better practice is to learn to use the histogram. The overexposure warnings (or "blinkies") can help on exposure, but they can be misleading compared to the histogram.

If the warnings don't show up, there's a tendency among many photographers to think the exposure is okay when shooting with RAW when it could be underexposed. I've seen underexposed images from some pros that have distinct problems in the dark areas because they rely too much on these overexposure warnings.

Overexposure.
Once you get excess exposure with too much of the histogram heading toward the right side, you also have tonal and color problems in later adjustments. You won't get added noise, but you'll get blocked-up, detail-less highlights that can be difficult or even impossible to deal with.

White Balance. Shooting on auto white balance won't cause any quality problems when shooting RAW, but it can create workflow issues. Auto white balance is very accurate today, but it can be fooled. I had a student on a trip to Peru who had a terrible time getting her camera to capture good color. Admittedly, she could correct that in RAW, but think of the disappointment from seeing images not quite right from a great location and the time required to "fix" photos later in the computer.

After suggesting that she use the custom white balance function of the camera, the images cleaned up nicely. Now she could review (and share with the group) photos that looked good immediately. In addition, she'd have less work to do on them when she returned home.

When you set a white balance in RAW, no pixels are harmed, but a tag of information goes with the file so that when it opens, it opens in the RAW converter with that white balance. You now have a specific point of reference to adjust from rather than the arbitrary and sometimes capricious white balance chosen by the camera.

Shooting it right for RAW can go beyond simple exposure. There's an excellent technique that helps in contrasty situations based on processing a RAW image twice (once for shadows, once for highlights), then bringing the two processed files together for a better overall tonal rendition than trying to process it all at once (see Linde Waidhofer's excellent article in the February 2004 issue of OP, "Expand Tonal Range Using A Single Image").

Sometimes a scene will have too much contrast for that technique, with tonalities beyond the capabilities of the sensor. Then you can expose twice while on location, exposing once for the shadows and once for the highlights. When in RAW, this technique offers an incredible amount of tonal and color detail that's impossible to capture in any other way. This will give tonal details like the old black-and-white masters achieved by using exposure and film developing to extend the negative's ability to capture brightness.

RAW has some amazing capabilities that only can begin to be discussed here. We'll come back to tips for processing the RAW file, but for now, just remember that RAW's full possibilities are enabled only when the image is thoughtfully exposed and captured by the sensor. RAW makes a good exposure better—and only means a lot of work with a poor exposure.

Editor Rob Sheppard's latest book is the PCPhoto Digital Zoom Camera Handbook, a guide to advanced digital cameras. He's leading a photo tour to Peru again this summer; visit the Palm Beach Photographic Centre website at www.workshop.org.

 


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