Noise and it's reduction

18/11/09

Unfortunately, noise is one of those areas that is very difficult indeed to explain in simple terms, and therefore l immediately apologise if the following tends to sound a little technical.

THE CAUSE : SENSOR NOISE

Each pixel in a camera sensor contains one or more light sensitive photodiodes which convert the incoming light (photons) into an electrical signal which is processed into the color value of the pixel in the final image. If the same pixel would be exposed several times by the same amount of light, the resulting color values would not be identical but have small statistical variations, called "noise". Even without incoming light, the electrical activity of the sensor itself will generate some signal, the equivalent of the background hiss of audio equipment which is switched on without playing any music. This additional signal is "noisy" because it varies per pixel (and over time) and increases with the temperature, and will add to the overall image noise. It is called the "noise floor". The output of a pixel has to be larger than the noise floor in order to be significant (i.e. to be distinguishable from noise).
 
THE EFFECT : IMAGE NOISE

Noise in digital images is most visible in uniform surfaces (such as blue skies and shadows) as monochromatic grain, similar to film grain (luminance noise) and/or as colored waves (color noise). As mentioned earlier, noise increases with temperature. It also increases with sensitivity, especially the color noise in digital compact cameras (example D below). Noise also increases as pixel size decreases, which is why digital compact cameras generate much noisier images than digital SLRs. Professional grade cameras with higher quality components and more powerful processors that allow for more advanced noise removal algorithms display virtually no noise, especially at lower sensitivities. Noise is typically more visible in the red and blue channels than in the green channel. The following example shows noise :


























NOISE REDUCTION
 
For the past four years I have spent hours researching methods to reduce noise from digital camera images. The key to noise reduction is to reduce or eliminate the noise without deteriorating other aspects of the image. Many freeware and even paid solutions negatively affect image sharpness, introduce wavy patterns in uniform surfaces and/or make them look "too uniform" (a bit like in a water painting).
 
I have used in camera noise reduction and that provided by my Workflow supplier, but none have been entirely successful. What l wanted was a nice straight forward and simple solution that even l could operate without too much technical knowledge, and which didn’t produce the results as outlined in the above paragraph.

Fortunately l came across a piece of software called Dfine 2.0 by Nik Software and this was supplied as a plug in to Lightroom. When loaded it therefore sits within Lightroom as a drop down menu item and can be easily and simply appplied to any image you chose. The following is the above image processed in Dfine 2.0 at default settings :


























Now whilst the result at default setting may not be perfect, it can be easily adjusted to higher levels in an effort to improve the image further.

However if there is one lesson l have learnt when dealing with noise reduction, it is that quite often, less is better than more.