What’s the Big Deal About Digital Noise?

Today’s Post by Joe Farace

Last night I was watching the film Three Days of the Condor for maybe the fourth time and was struck by one scene showing  photographs made by Faye Dunaway’s character. The camera does a close up of her prints and the first thing I noticed was the grain, probably from Kodaks’ 35mm Tri-X negative film. And then it hit me: Grain is an integral part of the picture but when it comes to noise photographers constantly complain about noise and try to eliminate it form their images. Why? Isn’t digital noise a natural part of the process, just like film grain. And everybody knows where noise in digital image files comes from, don’t they?

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Accumulative noise increases with long exposures made under low-light conditions and at high ISO settings, a typical night photography scenario. Noise is spread across the frequency spectrum but is more obvious in areas of underexposure. Noise varies with color and brightness and  is different for every digital camera but blue-channel noise is usually higher than in other channels. 

Digital noise has many internal causes too. Dark noise is produced by heat from the camera’s sensor during capture and is collected along with the data from light passing through the lens. Random noise is created by fluctuations within the camera’s circuitry or even  electromagnetic waves outside the camera. Signal noise is caused by fluctuations in the distribution of light striking an image sensor. Amplified noise is caused by using high ISO settings and is the digital equivalent of “pushing” film to achieve greater  sensitivity.

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There also several sources of external noise, including “pixel death” that’s more pronounced at higher altitudes. Then there’s the Taos Hum perhaps? Now before you think you’re reading the script from a rejected episode of “The X Files” think about this…

Geek alert: Sunspots are an intensely magnetic area on the Sun’s face that is slightly cooler than the surrounding photosphere. It may be because the magnetic field interferes with the outflow of solar heat in that region and appears a bit darker or not. Sunspots tend to be associated with violent solar outbursts and the number of spots varies on an 11-year cycle. Scientists analyze hot pixels to measure the effects of solar particles and cosmic rays on digital images and based on this, you could extrapolate that sunspots indirectly cause hot pixels due to the solar particles and cosmic rays they generate. Ask Stephen Hawking, not me.