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Tuesday, July 28, 2009

Noise: Lose It, Part I

Eliminate noise at the source


This Article Features Photo Zoom

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© 2009 John Paul Caponigro
1) Underexposed with shadow clipping
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© 2009 John Paul Caponigro
2) Normal is “underexposed”
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© 2009 John Paul Caponigro
3) “Overexposed” is optimal
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© 2009 John Paul Caponigro
4) Overexposure clips highlight detail
Noise happens. There’s always some degree of noise present in any electronic device that receives or transmits a signal. Though noise is unavoidable, it can become so small relative to the signal that it’s no longer visible. It’s all about a good signal-to-noise ratio (SNR). The image is the signal. The capture medium and the carrier medium each creates byproducts that we recognize as noise. The higher the SNR, the more the image overpowers the noise; the lower the SNR, the more the image becomes confused with noise. While some noise increases the apparent sharpness of images, the vast majority of noise found in images degrades quality. It’s best to avoid it.

There are a number of things you can do to reduce noise in your digital images at the point of capture.

• Use bigger sensors
• Use lower ISO settings
• Use faster exposure times
• Keep equipment cool
• Expose to the right



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5) Noise can affect tonal distribution
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6) Noiseless
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7) Noisy
© 2009 John Paul Caponigro

Knowing How Noise Is Produced Will Help You Avoid It
When it comes to noise, bigger is better. Bigger sensors have more light-gathering capacity, producing a higher SNR, or cleaner images. More isn’t necessarily better. Cameras with more photosites (yielding more megapixels) packed into smaller areas tend to produce a lower SNR, or noisier images. That said, a stronger signal doesn’t necessarily guarantee lower noise. It’s the relative amounts of signal to noise that determine how noisy an image appears.

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© 2009 John Paul Caponigro
8) Max signal/no floor noise
Higher ISOs amplify noise. ISO (International Standards Organization) is a descriptor that signifies absolute sensitivity to light. A digital sensor has one native ISO. Higher ISO settings simply boost the resulting signal. This is useful, but not ideal. When the brightness of the image is boosted, the noise is too.

Longer exposures generate more noise. Hot pixels become hotter. All sensors have a few pixels that heat up faster than others, producing brighter than expected values. Some even have a few dead pixels that never fire, producing only black pixels. During longer exposures, hot pixels are given more opportunity to heat up, growing brighter still; slightly hot pixels not visible at shorter exposure times become visible. As digital sensors age, hot pixels may become hotter and more pixels may become hot. Hot pixels produce a consistent fixed pattern of noise that can be recorded for given exposure times, making it easy to reduce.

 

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