Some cameras have a menu setting to turn noise reduction on or off. “Noise” is the random white or colored specks you sometimes see with long exposures. Noise may sometimes be referred to as “artifacts”, though technically these are something different.

The technical name for the noise reduction process is called “Dark Field Subtraction”. (Clearly, its clever marketing name is an improvement.)

Here’s how it works:

• With noise reduction on, your camera follows each exposure with another exposure of equal length. (With some cameras this only happens on exposures over a certain length, as long exposures are the ones that benefit most from noise reduction.)
• The second exposure is made by the camera with the shutter closed. It is a pure black exposure and is not saved.
• The camera analyzes this pure black exposure looking for any non-black specks (pixels).
• It then compares the location of any non-black pixels it finds to the same location on the previous exposure.
• If the pixel there is the same color, the camera analyzes the surrounding pixels and automatically cleans up the offending pixel based on the colors, brightness and tones of the surrounding area.

Noise reduction is not magic so it may not be able to clean up 100% of all noise, but it can make a notable difference.

As noted, noise reduction usually has little beneficial effect on exposures of less than one minute. That’s because you generally only get noise on longer exposures. It’s caused by your digital sensor heating up from prolonged use and some of the pixels registering “false positives”.

That is, some pixels report seeing light that they don’t really see. Once the sensor has cooled, the pixels will correct themselves.

Noise reduction also applies only to digital cameras. Film had its own equivalent but only certain types of film were affected and the exposure time thresshold was much longer. (Typically over an hour.)

If you want to really understand the mechanics of how your camera works, check out our ebook on the topic: