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Saturday, October 1, 2005

Watch Out For Unintended Noise


Noise is an important issue that's often overlooked in discussions about digital image processing


Noise in photos seems a little like plaque on teeth. Nobody wants to have it, the dental hygienist will say you have too much, and no one talks about it in polite company. While noise is a common problem in digital photography, you almost never see anything about it in the books on Photoshop or other digital subjects. About the only reference to it is the software that can minimize it.

For most nature photographers, noise is a problem and something to be avoided. There are a number of things that I'm seeing related to noise in today's photos that you may want to consider to get the most from your digital camera.

dh noisePeople are underexposing digital and getting more noise than they need. This underexposure means the image has to be brightened in the computer, which always results in more noise than an image shot with better exposure. In essence, underexposure is wasting some of the capabilities of the sensor and forcing it to give more noise.

The answer: Be sure to give adequate exposure. Check your histogram to ensure that your highlights are as far to the right as possible, which will result in the dark areas being exposed much brighter. Dark areas can be darkened, which will minimize noise, but if they have to be brightened, noise will be revealed.

RAW and JPEG both have noise. In some cases, shooting JPEG can be advantageous in terms of noise because the internal camera processing of JPEG files often works to reduce noise, especially at long exposures (more than 10 seconds). For normal shooting, while RAW won't reduce noise, it does help you deal with it. In Photoshop's Camera Raw, for example, you can reduce noise in the Detail Tab to good effect.

The answer: Shoot JPEG for very long exposures and check to see how you can reduce noise in RAW when processing it. Enlarge the photo so you can see the noise when reducing it, but be careful you don't overdo it, as this will adversely affect small details in the image.

Photoshop's Shadow/Highlight adjustment makes noise more apparent.
This relatively new tool for Photoshop seems to create magic by revealing detail in the shadows, but in doing so (and in the way that it does this), noise is emphasized.

The answer: Be careful in how you use the Shadow/Highlight adjustment. The default setting is way too high for most situations (it looks unnatural, besides causing noise problems). Try 30 for Amount, 20 for Tonal Width and 30 for Radius, and click the Save As Defaults button. This is a better starting point. Even then, be aware that you may have to make a compromise between revealing shadow detail and increasing noise.

Sharpening technique can cause noise problems. One serious limitation of Photoshop CS2's new Smart Sharpen filter is the absence of the Threshold control that comes with Unsharp Mask (USM). All digital SLR and compact digital cameras have some noise that always will show up in smooth-toned parts of a photo, such as sky or out-of-focus areas. If the image is sharpened indiscriminately, noise can be significantly emphasized unnecessarily so.

The answer: Set the Threshold of USM between 2 and 4 for most digital cameras (higher if the noise is strong). Check smooth-toned areas such as sky for this noise and see what happens to it as you move the Threshold slider. Use the 100% preview in USM to look at the noise. You can click with your cursor in the photo to show a new area in that preview.

Noise-reduction software works. This can be an important tool to have with your image-processing program. Photoshop CS2 introduced a noise-reduction filter, but to be honest, it doesn't work very well. I've been pleased with results from Noise Ninja (www.picturecode.com), Dfine (www.nikmultimedia.com) and Digital GEM (www.asf.com). Noise Ninja provides excellent analysis of the image for great noise reduction, Dfine offers an amazing set of tools for more refined control, and Digital GEM is probably the simplest and easiest to use of the group. These plug-ins work in Photoshop, Photoshop Elements and Corel Paint Shop Pro.

The answer: Use a noise-reduction plug-in. I recommend using a separate layer for this, as noise isn't always evenly distributed in a photograph. You may want to strongly reduce noise in a dark area, but then only use that part of the photo, rather than the whole thing that has had noise reduction applied where it wasn't needed. Either use a layer mask to isolate and use the noise-afflicted and processed portion, or erase parts of the noise-reduced layer that aren't noise-challenged.

Overprocessed noise can be as bad as too much noise.
Some photographers are so concerned about noise that they overprocess it with a noise-reduction program. This can cause several problems, including poor image tonality, colors that look plastic and loss of fine detail. In addition, the photo often will look less sharp, and not just from the loss of the noise, but also because sharp edges in the photo can be processed and blended by noise-reduction programs.

The answer: Be critical about how any noise reduction affects your photo. Look at fine color tonalities—do they still look rich and real? Look at delicate details such as tree branches—are they still strong? Too much noise reduction can make them look fuzzy.

Consider the good side of noise, too. If you make a big enlargement of your digital file, try adding a slight bit of noise to it after you've sharpened it. On some photos, you may find that the noise does nothing for the image, yet on others, it makes the photo look just a little sharper.

The answer: How much noise you add depends on the size of the photo, but try an Amount between 4 and 10 in Photoshop's Add Noise filter for images 13x19 inches or smaller. The high end will be noticeable, but the evened-out noise often is better than the more random nature of real noise and it may just make your photograph look sharper.

I hope this has given you a better perspective on noise and how you might affect it. The key is to be aware that it exists and that you have some options to control it.

OP editor Rob Sheppard's latest book is the PCPhoto Digital Zoom Camera Handbook, a guide to getting the most from advanced compact digital cameras.

 


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