Noise: The Silent Killer

I hate listening to static and commercials on my car radio, so I got satellite radio and a multidisk CD player. Too bad it's not as easy to eliminate noise from digital images. Even with a DSLR, I get noise every time I make a long exposure or shoot at a high ISO. With most 8 -- 10MP compacts? Forget it!


For those of us who once shot film, noise is as bothersome as film grain -- at least visually. Both tend to be more disturbing in shadow and midtone areas than in highlights, and both increase dramatically at higher ISOs. Film grain is fixed by the nature of the emulsion and processing, but digital noise can be produced by a wide variety of camera variables.
For starters, all electronic components in the imaging chain generate background noise, even on the best DSLRs. That's why even at a camera's base ISO setting -- usually between ISO 64 and 200 -- you may notice some noise in shadow areas and skin tones. You can reduce background noise by keeping your camera cool (an incentive to stop checking every shot in the heat-producing LCD monitor).
Other types of noise are harder to eliminate. These are caused by image stabilization systems, chromatic aberrations in the lens, white balance errors, exposure, and even the ambient lighting in a scene. (Most cameras generate more noise in the blue channel when shooting under tungsten lights, which is why we only use daylight-balanced HMI lights in the Pop Photo Lab.) High noise levels can reduce color accuracy and contrast, obscure details, and degrade image quality.
At higher ISO settings, most digital cameras turn on a noise-reduction system that acts by blurring the noise. The side effect is decreased resolution and detail. Usually, RAW image files contain slightly less noise than comparable JPEGs, since compression artifacts also contribute to noise. Most RAW converters allow you to fine-tune noise reduction in a file before saving it as a TIFF or JPEG, and there are also a number of good programs for reducing noise in JPEG images (again, with some trade-offs).
Until recently, we rated noise at different ISO settings by analyzing a 100x100-pixel area on an FBI SIQT v.1.0 test target. We exposed the target to get a brightness value similar to a caucasion skin tone (in LAB space, a 71L; in RGB space, an average 175 brightness out of 256) and listed the average of RGB and Luminance Standard Deviation values generated using Adobe Photoshop's Histogram control panel. Our rankings for those numbers (from Very Low to Unacceptable) can be found at www.PopPhoto.com or "How to Read a Camera Test" in the December 2005 issue.
However, analyzing a single gray patch doesn't always tell the whole story about how a camera handles shadow and highlight areas. To confirm our lab findings, we also study real-world photos.
And, starting with last month's review of the Pentax K100D, we are now using DxO Analyzer 2.0 software to analyze the entire grayscale on a standard GretagMacbeth Color Checker Chart. Our rating will still be based on the standard deviation of the grayscale patch with an average RGB brightness of 175. However, we will also post graphs at http://www.popphoto.com/ of noise levels for all values in the grayscale, as well as expanded noise analysis.
We hope that our improved noise analysis and charts online will help you make a more educated decision about the image quality of your next camera purchase, especially if you plan to shoot often in low light. But I'm still waiting for the equivalent of noise-cancelling headphones for my camera.

Full article here http://mcnamara.popphoto.com/the_mcnamara_report/2006/10/noise_the_silen.html#more
and much more here http://mcnamara.popphoto.com/

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Oh, and the PhotoPlus Expoin NYC - November 2-4, 2006 - ends today.
Be there as the ever-changing world of photography & imaging comes into focus atPhotoPlus Expo 2006— the premier event for innovative ideas,essential learning andunparalleled networking.
http://www.photoplusexpo.com/ppe/index.jsp

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