Preventing Noise at the Point of Capture

The interest digital photographers have in wanting to minimizing noise in their images is reflected in the importance given to noise test results in new camera reviews. The final evaluation of noise testing is usually summarized in general categories, such as extremely low, moderate, high, or unacceptable. These evaluations are based, in part, on the measurement of the cameras signal-to-noise ratio, abbreviated as S/N. The S represents the strength of the image-forming signal and the N represents the amount of noise. The higher the proportion of noise in the mix, the greater the chance that it will interfere with the quality of the image. The newest cameras, especially those in the professional class, are now producing low to moderately low noise with ISO ratings as high as 3200 and even higher in a few select pro models.

Digital noise is actually a catchall term for a number of different unwanted signals coming from various sources. An Internet search of the phrase digital noise in photography will bring up many technical articles for those of you who would like to delve into the physics of this subject. Most photographers, however, are concerned with practical answers to two main questions: What can I do to keep noise to a minimum when taking the picture? How can it be filtered out in the computer? Regarding conditions at the point of capture, there are three main situations that have the potential to produce greater noise levels: high ISO settings, long exposures, and underexposure.

Noise and High ISO: The prominence of noise at higher ISO settings has to do with the increase in sensitivity to light. Higher ISO functions are achieved in camera by amplifying the image-forming signal above the base ISO level, and that also means amplifying the noise. As a generalization, noise usually becomes a significant problem with point-and-shoot camera models at about ISO 800, or even ISO 400 with some models. With consumer level D-SLRs, that number is closer to ISO 1000, or perhaps even 1600. However, the reason for using a high ISO setting is to deal with low-light conditions, which is a second potential source for more noise due to a high degree of shadow areas where the light intensity is very weak. So, in many cases, some noise in unavoidable.

Long Exposures and Photosite Size: In low light, the additional exposure time needed to collect enough light to form the picture means that more noise signals will be recorded. This is true at any ISO setting. In addition, long exposures cause a heat buildup on the sensor that can produce a type of noise called dark current, or thermal noise. Many D-SLRs include a selectable noise reduction setting intended for long exposures. How effective this feature is will depend on the camera model. My experience has been that specialized noise filtering software will do a better job.

The size of the individual photosites is another contributing factor. The larger the photo site, the greater the number of light photons it can capture. This, in turn, means a lower proportion of noise signals. Small pointand- shoot cameras have tiny photosites on small sensors but with high megapixels counts, and that is one of the main reasons for their higher noise levels compared to D-SLRs with larger photosites.

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In the most comprehensive book on the subject, renowned author Joe Meehan details the use of both optical and digital filters in creative digital photography. Meehan walks both complete beginner and advanced amateurs through the various ways they can employ neutral density or polarizing filters to even out exposures or use soft-spot and diffusion filters to add atmosphere to portraits. There's also in-depth instruction on working with digital in-camera filters such as monochrome and sepia, as well as filters in image processing software (like Photoshop and plug-ins) to sharpen, distort, or create brush-stroke effects. This is the definitive guide that photographers have been waiting for!
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About Joseph Meehan

Joseph Meehan has been a professional photographer, writer and teacher for over 30 years. Hundreds of his images have been published in books and magazines all over the world. His style has been characterized by the New York Times as ". . . alive with color and sparkling with light." He served as the Senior Technical and New Products Editor of Photo District News for 12 years as well as writing articles and columns on photography and photographic techniques that appeared in such magazines as Popular Photography, Photographic Magazine, Outdoor Photography, and View Camera Magazine. Meehan also served as a contributing editor with the British Journal of Photography and the annual, The BJP Yearbook and is the former editor of the Photography Yearbook first published in Great Britain in 1935. His work has also been featured in all of these publications.

Joseph Meehan is the author of 25 books on photographic technique including: Panoramic Photography (Amphoto); The Photographer's Guide to Using Filters (Amphoto); Copying and Duplicating: Photographic and Digital Imaging Techniques (Kodak) and The Art of Close-up Photography (Fountain Press). His most recent books are: The Complete Guide to Filters for Digital Photography (Lark) and The Magic of Close-Up Photography (Lark). He also presents seminars and workshops at major venues such as PhotoPlus Expo, FotoFusion, DIMA/PMA, and MacWorld.

Joseph Meehan received his baccalaureate degree with honors from Columbia University and a Masters of Arts degree from Manhattan College. He lives in Salisbury, Connecticut, USA. www.josephmeehan.com

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