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Old Sep 27, 2007, 10:05 AM   #1
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Somebody said so, so I just tested the statement with my D50 with the lense being covered for 20sec at f/16, ISO200 w/o NR:


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Old Sep 27, 2007, 12:56 PM   #2
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I'll copy my answer from this recent thread where someone was asking about the noise reduction feature on a D40.

Note that camera/CCD temperature, ISO speed and exposure time can also impact how many hot pixels you get and how hot they are. The noise reduction should help at settings where you do get some at longer exposures.

http://forums.steves-digicams.com/fo...mp;forum_id=58

That feature on your D40 is not for typical noise you'd see a higher ISO speeds (although some models do have a feature designed for that now), and the reason it takes so long is because it's taking two photos with longer exposures (one is with the shutter closed).

It's designed to remove hot pixels you'd see on longer exposures, not pattern type noise.

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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Old Sep 28, 2007, 12:11 PM   #3
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Thanks Jim for the very detailed explanation!

In the sample I attached, I could not see any hot-pix for 20-sec exposure and with the NR being turned off.

Would that mean my D50 is immuned from hotpix, OR Nikon simply masks them off ???
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Old Sep 28, 2007, 12:32 PM   #4
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Try a longer exposure (i.e., bulb mode) and make sure Noise reduction is off and you'll probably start to see a few, especially if the camera is warmer.


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