John Paul Caponigro's "Digital Noise"

March 27, 2011

John Paul Caponigro's "Digital Noise"
In your test subject, include a smooth field of color where noise will readily be displayed…


While some digital photographers will use noise for creative effects, most prefer to keep it to an absolute minimum. Noise in digital files can be produced by higher ISO settings, long exposure times, and under-exposure, or any combination of these. Avoid them when you can. It’s best to reduce noise at the point of capture, rather than using software in the computer after capture. But, when an image contains noise, you can reduce it in your computer, sometimes dramatically.

A little testing makes reducing noise after capture easier, something you can automate in many cases.

Do you really know what to expect from your camera in different situations? If you run some tests to see how much digital noise your camera generates at different settings, you’ll be much better equipped to make decisions when you’re photographing actual subjects. Keep in mind that different digital camera models, even from the same brand, will often generate different levels of noise. So if you own different camera models, it’s advisable to test each one, and be familiar with their differences.

In your test subject, include a smooth field of color where noise will readily be displayed (texture may camouflage noise). Include a contour along an area of contrasting color. A horizon against a blue sky may do. Two sheets of colored paper certainly will. As noise produced by sensors is typically red, green or blue, non-primary colors such as yellow, orange or purple may display noise more readily, but a neutral value, such as a gray card, is a sure bet.

Test all three factors that influence noise

1) ISO Settings

The first and perhaps most obvious variable to be in control of is your digital camera’s ISO settings. Shoot the same subject at different ISOs, from the lowest to the highest. You can generally use full-step ISO increments, such as 100-200-400, this will be sufficient to give you a sense of how ISO affects noise in your files.

2) Slow Shutter Speeds (long exposure times):

Test what happens during long exposures. Photograph the same subject, making exposures with a wide variety of slow shutter speeds. You’ll probably need to do this in low light or at night to effectively extend exposure times. Start with exposures about 1 full second, and double each exposure from there. Be sure to adjust your lens aperture as you lengthen shutter speeds, to keep final exposures constant.

After the 30-second limit of most digital cameras, use an electronic remote switch and the Bulb setting, and keep doubling exposure times (1 minute, 2 minutes, 4 minutes, etc). Some digital cameras begin to pick up noise when exposures exceed a few seconds while others won’t display significant noise until exposure time extends beyond several minutes. Many digital cameras generate noise dramatically after a specific threshold has been met. Find the duration required to meet this threshold on the camera you are using. Avoid making exposures longer than this whenever possible.

3) Under-Exposure

Test how much noise is generated if you under-expose an image. Smooth mid-tone areas and deep shadows are especially susceptible to noise build-up, especially if you under-expose. This is because they are produced with fewer photons from each pixel, and recorded with fewer bits of data. Find a subject with smooth, non-textured middle tones and some shadow areas, and shoot it at normal exposure levels and then deliberately under-expose the scene to determine the point at which significant noise becomes present in shadows. Make a note of how the tones appear in your images’ histograms, and whenever possible weight histograms above this level, but do not create a bigger problem by intentionally over-exposing and clipping highlights in an effort to avoid shadow noise. To be thorough, test this for all full-step ISO settings (ISO 100-200-400, etc), which also impacts noise.

Viewing the Results:

Don’t judge results by simply viewing the image on your camera’s LCD monitor; you’ll need to actually view the file on your computer to begin to appreciate the results. For RAW files, be sure to actually process the RAW image and view it in an image-editing program such as Adobe Photoshop™ or similar — don’t rely on an enlarged preview in the software you use to actually process your RAW files. The preview is based on a low-resolution, embedded JPEG image that’s actually part of the RAW camera file, and isn’t a reliable indicator of noise effects.

There are two kinds of noise to be mindful of: Luminance (light/dark) and Chrominance (colored)

Luminance noise is the gray-to-black noise you see in a magnified view of an image with visible digital noise. Its “salt and pepper” look actually contributes to the detail in the subject. Therefore, tread lightly — be cautious when correcting luminance noise. All routines for reducing luminance noise compromise sharpness, some less than others. A little luminance noise reduction can be accomplished with most RAW converters. A lot of luminance noise reduction is best accomplished with third-party software programs dedicated to this function, such as Noiseware™, Noise Ninja™, and others.

Chrominance noise, which can occur simultaneously with luminance noise, is the pastel-colored specks of noise you may see in a magnified view of a subject. It’s often most visible in smooth, mid-tone areas (such as smooth sections of skin in a portrait). Reducing chrominance noise can be accomplished without compromising image sharpness. Chrominance noise is easily corrected with most RAW converters. It can be tempting to over correct for noise, so be conservative. In particular look for a reduction of saturation — too much chrominance noise reduction can begin to produce color shifts and changes in color saturation levels. Areas that are most susceptible are found along contours with a relatively high degree of contrast, particularly in hue. If chrominance noise is over-corrected, clearly seen bands of reduced saturation may be produced along contours.

Recent Canon EOS cameras, such as the EOS Rebel XSi, EOS 40D, and Mark III series models have a built-in setting to reduce chrominance noise, called High ISO Noise Reduction. It’s especially effective for users who shoot JPEG images. Other brands of digital SLRs sometimes have similar features built-in.

Reducing digital noise in the computer with RAW conversion software:

If you shoot RAW images, a number of options are available after the shot is taken to further reduce digital noise levels. Most RAW file converters, such as Adobe’s Camera Raw™ software (part of Adobe Photoshop CS3 and Photoshop Elements v. 6), as well as Canon’s Digital Photo Professional software, have tools to reduce the effect of noise in your files.

Open the files and viewing them at 100% magnification, determine noise reduction settings in the RAW converter of your choice.

These settings can easily be saved for future use, with what Adobe calls “Presets”. In Adobe’s Camera Raw™ (ACR), look for the tiny folded page icon under “Presets”, click on it, and in the New Preset dialog box that appears, check any adjustments you’ve made, including Luminance Noise Reduction and/or Color Noise Reduction.

In Adobe’s Lightroom™ software, click the + sign in the Presets palette (left side of the screen), and check Noise Reduction > Luminance and/or Color. With either Adobe software package, give the new Preset an appropriate name, save the Preset file, and now you can apply those same changes to future images.

 

Canon’s Digital Photo Professional (versions 3.2 or higher) offers similar capabilities.

Any adjustments you save are called “recipes”, and you can apply them to any subsequent Canon RAW files you process in DPP. After you make your adjustments and any noise reductions, choose Edit > Save recipe in File…. Give the adjustments a meaningful name, and pick a destination to store the “recipe” in (a folder on your desktop should be fine). Then, you can apply it to future images using the Edit > Read and Paste Recipe from File… command. (You can save as many different recipes as you like, to cope with various types of RAW processing needs.)

Photographers who prefer Adobe’s RAW conversion software will find these settings can be easily applied to other images made with the same camera and lens combination. In Adobe’s Camera RAW or Lightroom, any memorized Presets will be listed on-screen; simply select the appropriate one for the image(s) you’re working on. As just mentioned, Canon’s high-end Digital Photo Professional software lets you easily apply saved settings to new images as well.

A little testing initially will save a lot of time later; your testing might even save someone else time!

John Paul Caponigro is an internationally respected fine artist, a member of the Photoshop Hall of Fame, author of Adobe Photoshop Masterclass, the DVD series R/Evolution, and a renowned Canon Explorer of Light. Learn more about him and his work at The Canon Digital Learning Center, or at www.johnpaulcaponigro.com

The CDLC contributors are compensated spokespersons and actual users of the Canon products that they promote.

All images are copyright JP caponigro, Erika Silverstein

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