This is how I do dark-frame subtraction with my D1 time exposures:

1) Shoot a dark-frame with lens cap on using the same exposure time as the image.

2) Open image and dark-frame files in Photoshop (don't edit anything before this step).

3) "Select all" and "copy" the dark-frame.

4) "Paste" on top of the image frame (don't drag the dark frame or you will have alignment problems).

5) Select the top (dark-frame) layer and set the opacity to 50%, and the layer mode to "difference".

6) Flatten the image and it's done.

With 20 second exposures toggling the top layer on and off before flattening shows a dramatic effect. I found that lowering the opacity of the dark-frame layer works better than using any blur filters. You can fix old exposures where a dark-frame wasn't made by using a recent dark-frame--since long exposure noise appears in roughly the same place on a given CCD. Remember that noise increases with temperature. You can use dark-frames made at higher temperatures or longer exposures than your image--just reduce the top layer opacity until everything looks good. If you don't reduce the dark-frame layer opacity enough, you will have dark spots in your image highlights. Always try to use ISO 200 with the D1, because high-ISO noise is more random, and can't be removed with this method.

Good luck,
Fred


Paul Caldwell wrote:
> I wanted to ask some of the Pros out there their opinions on this.
> Unfortunately the 3 books I have on photoshop don't really seem to cover
> it and I know it has been written up alot in the various forums. I am
> looking for two things.
>
> First I want to better understand just what it is supposed to remove. I
> have read all the posts and some people seem to think it is to remove
> random noise and stuck pixels and others seem to think it is just stuck
> pixels. From what I have read about the Canon D30's long exposure noise
> reduction setting, it would have to be removing both stuck pixels and
> random noise.
>
> As far as the actual process, I understand the method, take a 3"
> exposure then take another 3" exposure with the lens cap on. Its the
> next part I am confused about. How to load the two images in photoshop,
> and then do the calcuations to remove the noise from the image with the
> totally dark image. I would greatly appreciate any ideas or help on
> either subject.
>
> Thanks in advance
> Paul
> pcaldwe@aristotle.net