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How to use noise reduction in camera
Techniques
24 October 2007 12:23
Slow shutter speeds are great for conveying subject movement and when you want to use a small aperture. However, long exposures also suffer from more ‘noise’ – any unwanted or incorrect information contained in the image. This is normally visible as variations in the colour, tone or brightness of pixels that should look uniform.
Most DSLRs offer a mode that can help to reduce this effect for exposures of one second or more. This is known as long exposure (or blank frame) noise reduction (NR).
After you’ve taken your long exposure, the camera takes a second exposure (known as a blank frame) of exactly the same length as the first, but without opening the shutter.
As this second image doesn’t contain any visual data (it’s blank), each pixel should give the same uniform tone, colour and brightness. Any variation is analysed by the camera to measure the noise present in this blank exposure. The camera then uses this information to remove noise from the original image.
When you use the noise reduction feature you’ll find that the camera locks up for a time after you’ve taken the shot. This isn’t too much of an issue with exposures of 1 or 2 seconds but waiting for 30 seconds in the dark to take your next shot can seem like an eternity, especially with moving subjects. This is the price you have to pay for using the feature though, so just be prepared to wait it out.
Noise is generally caused by pixels giving slightly different readings for the same exposure. The effect is at its worst when you’re using long exposures or high ISO settings because the signal is being amplified to produce the image. Noise will always be most evident in even-toned areas, such as skies, as detail tends to mask its effect. It’s not all bad news though, as noise gives a similar effect to grain in high-speed film, so you can use it for creative, impressionistic effects.
You’ll find the noise reduction mode in the camera settings menu. Scroll down until this option is highlighted then press the OK or select button.