Dark frame subtraction is meant to deal with damaged pixels. Noise usually visible in pictures is random in nature. This means two pictures after each other will have different noise. Another property is that it is zero on average. So with enough pictures taken from exactly the same point and with the proper layering technique, you can average out noise to a fashion. This is described elsewhere on the list too.
I'm puzzled that you claim much improvement by substracting one random signal from another....
--- In PanoToolsNG@yahoogroups.com, "Roger D. Williams" <roger@...> wrote:
> Hi, Ken.
> On Thu, 16 Apr 2009 18:23:24 +0900, Ken Warner <kwarner000@...>
> > Hi Roger,
> > If I had a better camera, I might be able to find that zone between
> > blown out and properly exposed -- if there is such a thing -- to get
> > into the recoverable highlights zone. It is a very delicate adjustment
> > of EV and even viewing test shots doesn't seem to allow me to find
> > that area. My LCD is small and shots don't look quite the same on
> > the LCD as they do on my computer.
> Yes, I have about given up on relying on the LCD to tell me what the
> image will look like on my computer. The histogram displays are a much
> better guide, and I get the best results when my highlights reach into
> the last quartile on the right which normally indicates "blown,"
> and certainly looks like it on the LCD. However, when I get into the
> RAW developer, and introduce a stop or two of "highlight recovery" it
> brings them down beautifully, while I can use "shadow recovery/boost"
> to bring back up the shadow areas.
> > On the topic of noise. UFRaw has darkfield subtraction where if you
> > shoot a frame with the lens cover on you will get just the sensor noise
> > which can be subtracted from a recent image. That works real well to
> > reduce noise almost to nothing even on low light days and situations.
> > Even on my cheapo camera...
> That sounds really cool. Is it unique to UFRaw? I haven't come across
> it in anything else I've used.
> Roger W.
> Work: www.adex-japan.com
> Play: www.usefilm.com/member/roger