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Noise” in the context of digital photography, is random unwanted digital data that becomes converted to colour and/or brightness levels that do not correspond to the original optical image.

All digital images contain noise – the issue is when noise levels become high enough to have an adverse impact upon image quality.

During exposure, the image sensor is electronically activated to record the light signals falling upon it. However even in the absence of any light, the sensor will generate random data - the longer the sensor is activated the greater the quantity of this random background “noise”.

At the end of the exposure recorded light signals are amplified before being converted to digital data – any background noise is also subjected to this amplification.

In simple terms – the greater the quantity of light that is recorded compared to the inherent background noise , the less the adverse effect on the quality of the image.

This light signal versus background noise is known as the signal to noise ratio.

Listed below are the various factors that result in a low signal to noise ratio and therefore result in noisy images.

  • Longer exposure times. The longer the exposure (slower the shutter speed) the greater the duration that the sensor is activated and so the greater the cumulative effect of background noise.
  • Small physical sensor size (not the number of mega pixels!)The larger the sensor area, the larger the aperture diameter at a given f number – larger aperture diameter equals greater light area through which light can be collected for a given shutter speed – hence a higher signal to noise ratio and therefore lower noise in the image.
  • Small Pixel size. The larger the physical pixel dimensions the greater the signal to noise ratio. So tiny sensors with a high pixel density (small pixels) will not necessarily produce better quality pictures than sensors of the same size, but with fewer pixels.
  • High ISO setting. ISO setting affects the degree of amplification of the light signals (and the noise) picked up by the sensor. The higher the ISO the greater the amplification. So if a sensor is prone to a low signal to noise ratio , then in low light situations when the quantity of light is even less, the problem is exacerbated. This is where larger sensors as found on Dslrs with their higher inherent signal to noise ratios offer considerable benefits, as a much higher ISO can be tolerated before noise becomes problematic.
  • Camera processor. Each manufacturer has its own proprietary method of converting data from the sensor into the digital image data – the conversion process will attempt to recognise noise and eradicate it – some manufacturers systems seem to cope with this better than others.

Noise reduction

Digital noise is a fact of life. There are however various post processing options that can be used to reduce noise in images. Adobe photoshop and camera raw both have noise reduction filters and there are also specialist “plug in” software programmes designed specifically to reduce noise.

Exposure settings

Noise is most prevalent when light data is scarce – so it follows that the darker areas of an image are likely to be most noisy. So brightening shadow areas to correct exposure in post processing will create noisier images compared to darkening slightly over exposed images. Care needs to be taken not to over expose to the point where highlights are “clipped” as then there will be no image data to recover.


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