On 18 Nov 2005 at 9:56, Bruce Dayton wrote: > Hello Don, > > The longer the sensor is on, the more thermal noise that is generated. > So the second frame may record the thermal noise and remove that from > the first exposure. This would give you a cleaner black image. The > assumption the camera can make is that the second exposure is supposed > to be pure black, so anything that shows up otherwise should be > considered noise and can be removed from the first frame.
The in camera NR really isn't designed to tackle thermal noise, its label is a bit of a misnomer actually. Dark frame subtraction can only effectively combat fixed noise (error) sources like stuck or hot pixels, thermal noise frame to frame will be of course essentially random in nature. What the subtractive NR does seem to do though (apart from negating stuck/hot pixels) is normalize the sensor response, I find in long time exposures with NR on the noise appears more consistent across the frame. Cheers, Rob Studdert HURSTVILLE AUSTRALIA Tel +61-2-9554-4110 UTC(GMT) +10 Hours [EMAIL PROTECTED] http://members.ozemail.com.au/~distudio/publications/ Pentax user since 1986, PDMLer since 1998
