« on: August 02, 2011, 11:55:52 AM »
I just checked my 7D... long exposure NR is set to OFF.... still no noise
I'm glad you don't see noise in your shots in your applications, but it doesn't mean it's not there. Noise is present in every image, but the amount obviously varies, as does the impact, and the latter is certainly dependent on the application (cropping, on-screen viewing, small prints, large prints, pixel peeping, etc.).
Long exposure noise is different than high ISO noise, in that the former is reproducible for a given exposure time and sensor temperature, whereas the latter is random. Thus, long exposures can have NR applied quite effectively in-camera (the computation is very simple, literally just subtracting the e- recorded at each photosite in the dark frame from the same photosite in the image). NR for high ISO is much more computationally intensive, and a computer will do that better than the on-board chipset. Also, post-processing NR for long exposures is not the same as NR for ISO noise. Unless you plan ahead and shoot a dark frame, you can't do long exposure NR in post. Many astrophotographers keep libraries of dark frames at various exposure times and temperature conditions and apply those in post, because it speeds up the image capture (i.e. you can just take a set of 30 s exposures abck to back, instead of the in-camera process of 30 s exposure followed by a 30 s dark frame).
I will say I haven't attempted astrophotography in almost 6-7 years (and even with that I shot with my medium format camera)... I guess I suppose having the the camera on bulb for a few minutes at a time if not longer with humidity and cold temperatures (situations which digital cameras struggle and have to work harder) then noise would be visible... I'm lucky enough to be in a dry climate with warm summer nights so I wouldn't have the same situation humidity wise as if I was sea level at lets say san fran or the midwest... (I had to do a shoot in chicago in which the humidity was so high (99%) my 30D at the time quit working until i got it in an air conditioned room to cool down and dry out.
Most of my long exposure stuff is for things like fireworks/lightning and high end architecture at twinight and usually (with digital) I dont get exposures longer than 30 seconds and typically noise at low iso is no problem for what I use it for. (film with reciprocity my long exposures get a heck of a lot longer to make up for it)