Digital BW, The Print

Can you explain how you do this? 
  ----- Original Message ----- 
  From: Jeff Medkeff 
  To: DigitalBlackandWhiteThePrint@yahoogroups.com 
  Sent: Saturday, July 02, 2005 6:36 PM
  Subject: Re: [Digital BW] Artifacts with Digital images


  . So dark noise on that camera gets significant 
  after several minutes of exposure time. I'm more concerned about photon 
  noise until out past five minutes or so, at which point I start to apply 
  calibration frames to my images to deal with dark and bias noise.



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