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Monday, April 30, 2007

April 30, 2007 HelpLine

Controlling Noise

Labels: HelpLineHow To

Q) What's the best way to control noise when shooting? I recently took some photos of the Northern Lights and the results were very noisy.

Steven J., Via the Internet


A) Headphones? (Sorry, couldn't resist!) There are three ways to control noise when capturing images. The first is to use the lowest ISO setting you can. The ISO setting is commonly said to be a way of controlling the sensitivity of your image sensor. In reality, your image sensor only has one sensitivity.

When you change the ISO setting on your camera from a low setting to a high setting, the camera amplifies the small signal that comes off the photosites on your image sensor. Any time you amplify a signal, you also amplify any noise contained in the signal. So keep your ISO setting low and use your shutter speed and aperture setting to boost the light reaching the sensor.

Obviously, there will be times when you'll need to use a long shutter speed to capture some images. In particular, nighttime shots might use exposures that run into the seconds-like your Northern Lights shot. In the case of a long shutter speed, check to see if your camera has a noise-reduction mode for long exposures. This technology takes a virtual "second" exposure after the camera is finished capturing the first shot. This second exposure is made with the shutter closed so the image sensor doesn't receive any light. The exposure lasts as long as the original exposure and is used to capture the noise that's inherent in the image sensor. The image processor then uses this virtual image to subtract noise from the original image.

Since this second exposure lasts as long as your original exposure, make sure you have enough power to allow the process to happen. I've done some star-trail exposures that entail a 30-minute exposure. With the noise reduction, the total exposure is one hour, so battery life (or an AC adapter) is critical.

The last way to control noise is to expose properly. With the Northern Lights, you don't have many options, but with normal scenes, I've seen noisy pictures when people shoot with the "I'm shooting RAW so I don't have to worry about exposure" philosophy. Use your histogram to make sure you expose properly, and you'll be able to get wonderful shots with low noise.



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