My understanding is that it is really a dark frame subtractive process. The basic idea is that second exposure is taken by the camera of black/no image. The sensors that are susceptible to heat/noise will show up in something other than black. Then with this map of noise the camera can fill in those values with neighboring pixels from the first image. This effectively gets rid of noise introduced by the long exposure. The time it takes to do this is the same time as the original exposure plus the processing time.
In practice it works pretty well. -- Best regards, Bruce Friday, September 10, 2004, 8:58:48 AM, you wrote: SB> What if something has moved in the frame ... not everything one photographs SB> remains perfectly still all the time, and not always is the camera securely SB> mounted on a stable, immovable platform. That's my guess .... SB> Shel >> From: Nenad Djurdjevic <[EMAIL PROTECTED]> >> To: <[EMAIL PROTECTED]> >> Date: 9/10/2004 8:42:22 AM >> Subject: Re: long-time exposures with *istD >> >> Jostein wrote: >> >> > The noise reduction is actually an additional exposure of the same SB> length >> as the one >> > you took, but with the shutter closed. This gives the camera info about >> the noise that >> > it can subtract from the real exposure. >> >> My understanding is that noise does not necessarily conform to a pattern SB> but >> is random. If that is the case wouldn't it be more logical for the camera >> to make another exposure with the shutter open, compare the two shots and >> delete anything that did not occur in both photos? (Please forgive my >> ignorance if I have missed something obvious ;-).
