Long Exposures & noise... ISO vs shutter time
I use a B+W 10 stop filter for long exposure photography and have been wondering about the relationship between exposure length and ISO in terms of noise generated in the resulting image.
Is there a reciprocal relationship here? Would you get the same amount of noise added to the image by doubling the exposure time as you would doubling the ISO?
Just wondering because I find in some situations I am shooting its not really much of an advantage to use, say a 4 minute exposure over a 2 minute exposure (in terms of the aesthetic look of the image) but the exposure is reliant on the 4 minute shutter speed because of the available light. So I guess i'm saying I can use a higher ISO to achieve the same effect (and less waiting around time) but which option would be best purely in terms of image quality (noise)?