I just found a really effective way to do DIY removal of "Dark Channel Noise," the white speckles you get when you take long exposures.

I also posted this to my "home" forum, the Nikon SLR Talk forum. I figured it would be interesting to both audiences.

The Nikon D100 and other cameras have an automatic noise removal function, but those of us with D1 family cameras don't have that option.

In order to remove the dark channel noise you need to take a "dark frame," or a picture with the lens cap on, within a short time (a minute or so) of taking the picture you want to fix. The dark frame should be taken with the same settings as the image you want to improve (same ISO, shutterspeed, aperture, white balance, etc., etc.) This technique will work best on raw or TIFF images, since JPEG compression will tend to shift around the pixels some.

There's a learning tutorial here on DPReview called "Night Spots" by Peter iNova that gave me the technique. I've modified it slightly, but the basic idea is his.

In the past, I've layered the dark frame with the image I want to fix up and used "difference" blending. That gave an improvement, but it wasn't great.

Peter iNova's technique involves using the "dust and scratches" filter on a custom mask that you generate using the dark frame. Here's how you do it. I'll describe it using Photoshop 7.0 and the Nikon NEF plugin on a Nikon raw file, although there's very little that's specific to Nikon's files other than the step of specifying the WB and EV settings as you open the file.

It does a good job of blending the hot speckles of dark channel noise into the surrounding image, without degrading the rest of the image very much at all. That's because the process only acts on the spots, and doesn't affect the other parts of the image at all.
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Open both images using the same settings for white balance, exposure compensation, etc.

Select the "move" tool and shift-drag the dark channel image onto the image you want to fix. This "pin registers" the two images so they line up perfectly.

Make sure the dark frame is visible (the image will look black, with speckles of white that are visible at high magnification). Go to the channels palette and command-click (control-click on the PC) on the thumbnail for the RGB channel. This will convert that the dark frame into a selection. White pixels will be fully selected, black pixels will not be selected at all, and in-between pixels will be partially selected based on their luminance.

Now go back to the layers palette and hide the dark frame layer. Make sure the layer with picture you're trying to edit is the active layer. Then bring up the "dust and scratches" filter. I've found that a setting of about 3 pixels seems to give good results for a full-resolution 5-6 megapixel image.

Here is a sample image I took to test out this technique. The picture is nothing special -- just a shot of the darkened inside of my pantry. Both the original picture were shot at 25 seconds, f/20, ISO 125, in Nikon raw format using Adobe RGB colorspace. I didn't make any attempt at an accurate white balance, so there is a color cast to these pictures that I decided to leave. The pictures are 100% crops of the before and after pictures, plus a crop of the dark frame used to correct the image:

Original, noisy image:
http://www.pbase.com/image/24136220.jpg

dark frame showing dark channel noise:
http://www.pbase.com/image/24136218.jpg

fixed image:
http://www.pbase.com/image/24136219.jpg
--
dpreview and PBase supporter.


http://www.pbase.com/image/14555687.jpg

http://www.pbase.com/duncanc