Photoshop Basics – noise reduction

No camera is perfect. If we push it to the limits of iso and low-light scenes, we are going to experience the number one weakness of a digital camera – NOISE.

In camera noise control

When you use the long exposure noise reduction setting on a D-SLR, it takes the same shot with the shutter closed and then takes the information away from the original exposure. This is why you will have to wait the same amount of time again after taking the shot this can be an inconvenience if you want to keep shooting. You can save this time and do this yourself with Adobe Photoshop or similar photo editor. If you are out taking a dozen 20 second exposures, turn off the in camera noise reduction and just shoot one frame for the same exposure time with the lens cap on. In photoshop use the ‘difference’ blend on the layers palette to reduce the noise in each of your pictures. This technique takes care of sensor hot-spots.

General noise reduction

This tutorial aims to show you how to correct for general noise using photoshop.

First we will look at the causes of noise and what can be done to prevent/ reduce it in the first place.

Noise in digital images is usually due to the fact that we have a highly sensitive part inside an electrical appliance. Noise is mostly electrically produced and unacceptable noise is where the sensor picks up more electrical interference than light information. Generally speaking, the larger the sensor i.e. full frame DSLR will cope with noise issues so much better than small compact digital camera sensors.

Unacceptable noise

The following situations can produce unacceptable noise.

  • Long exposures – heat from the activated sensor produces coloured dots
  • Low light – when the electrical interference is stronger than the light in the scene
  • High iso – the light information is amplified to make the sensor more sensitive

Steps can be taken to avoid noise occurring in these cases such as ensuring that there is enough light available to use a reasonable iso and exposure time, but sometimes we want to shoot in low light or we are unable to provide another light source.

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