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Old Sep 24, 2006, 10:17 AM   #1
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Join Date: Sep 2006
Posts: 28

All the manual says is "on or off", not very telling, and unmentioned in any reviews I have read.
Is it an untold secret, or what? Better than PP?

Experience please, and as usual Thank You in advance!

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Old Sep 24, 2006, 10:44 AM   #2
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Location: Savannah, GA (USA)
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That feature is not for typical noise you'd see a higher ISO speeds.

It's designed to remove hot pixels you'd see on longer exposures.

Hot pixels are when photosites record a higher value than they should, usually with longer shutter speeds.

Virtually all CCDs will have them in some conditions. The longer the shutter is open, the more likely a photosite won't be linear with it's fill factor, and it will record a higher value faster than it should.

You just don't see them with most newer cameras because of built in noise reduction to subtract them out on longer exposures (if you have it turned on), since the ones that occur at typical shutter speeds have already been mapped out at the factory.

With most cameras, it only engages on photos taken longer than 1 second (where hot pixels usually show up).

The way it works is by taking two photos. The first photo is of your subject. Then the camera automatically takes a second image using the same settings with the shutter closed.

The camera then notes the positions of any hot (bright) pixels in the "dark frame" (second) exposure, and maps them out of the actual exposure by interpolating values from surrounding pixels to replace the hot ones.

It knows where to find them because the dark frame exposure was taken at the same time, using the same settings, with the camera at the same temperature. In most cases the hot pixel locations will be the same in both images using this technique. But, the longer the exposure, the greater the chance it will miss some of them.

This type of noise reduction is known as "Dark Frame Subtraction".

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