Frantisek wrote, > Hi, > anybody knows what this technique is? I heard E-20 digicam uses > darkframe substitution to reduce noise, any pointers on it in > photoshop?
Hi Frantisek, Mark Roberts has given a very good answer to your question. I've got some additional information that's related, although not exactly an answer to the question you asked. Hope it may be of some use. One of the problems with, e.g., widefield astophotos (and also with widefield night scenes in general) is due to the degree of light fall-off with wide-angle lenses -- the raw image is much brighter in the center than at the edges. For conventional photographic imaging of things like nebulae, galaxies, and other low-contrast, dim, extended objects, the change in density across the picture (from center to edge) can wash out contrast on these dim, extended objects. A recent article in Sky & Telescope magazine describes how to make what is similar to a darkfield image (well, sort of), do background subtraction to compensate for the light fall-off, and rescue subtle details present in the dim subjects you're trying to photograph. It's in the September 2001 issue on pp. 116 - 120. The title is "Fixing Vignetting in Astrophotos", and it's written by Sean Walker, the advertising production manager at S&T. (It's really about fixing light fall-off, not vignetting -- I've always regarded these as two entirely different things.) Anyway, the article isn't on the S&T web site, but you might be able to get a copy of the article from the author directly, if you don't have access to recent S&T back issues. His e-mail address is: [EMAIL PROTECTED] The technique is simple enough. Hopefully my explanation will be clear. To create this background image, you take your raw image, then use the cloning tool to erase all of the stars. Thus, each bright star gets covered by some dark background sky in its immediate vicinity. After erasing all the stars, you need to blur the image a bit to make it nice and uniform. When you're done, you have a featureless image which is brighter in the center and darker along the edges. Save this background image as a separate file. You then subtract this image from the original raw image, and voila! Light fall-off is gone, and contrast of your dim objects is greatly enhanced! I tried it with some startrail exposures I'd made during the Leonid shower, and the results were pretty amazing. You can do the same technique with digital imaging; that is, subtract a true "darkfield image" -- like Mark Roberts describes -- to compensate for dark current and read noise of the CCD itself, then subtract the type of background mask I describe to account for light fall-off. The principle is the same. I know this really wasn't your original question, but I hope it helps nonetheless. Good luck with your post-processing. Bill Peifer Rochester, NY - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org .
