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GigaS27 Sep 5, 2007 8:42 AM

Got a question about the noise reduction funtion thats built into camera, i noticed it takes a really long time in order for me to take my next shot. Now sice so many post processing software out there have the ability to do noise reduction, can i leave this function disabled in the camera and rely on the post process? It would def. save me alot of time when shooting pictures...


JimC Sep 5, 2007 9:56 AM

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".

Bob Nichol Sep 5, 2007 10:03 AM

The technical term is "dark field subtraction". If you take a 5 second exposure, for example, the camera takes a second 5 second exposure with the shutter closed then inverts this black image and merges it with the real image taken previously. Any hot pixels should now be removed or minimized as a result. Works well if you always get the same hot pixels!

Most cameras should have a menu item to disable this function.

Hot pixels show up more during long exposures, 1 second or more, and are bright spots in the picture that shouldn't be there.

<JIMC got the answer in before I could!>

GigaS27 Sep 5, 2007 10:57 AM

Ahh IC what it does i guess i should turn it on anytime taking long exposure shots? and turn off and normal shooting??

Or would you guys just recommend to leave it turned on at all times...cuz even normal shooting seems like it slows me down a bit, but i could be wrong.

JimC Sep 5, 2007 1:25 PM

Just leave it on. It should only engage on long exposures.

GigaS27 Sep 6, 2007 7:29 AM

Thank you

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