I am using a nikon d5600. When i turn on the Long exposure NR, the camera takes as much time after the photo to process it, as the actual photo duration . I understand that this is because it takes some kind of black empty photo during this process.
So, if i shoot a 10 or 15 second photo, and then the camera starts doing the noise reduction. Can i move the camera i.e. change the composition during this processing phase, or do i have to keep camera in same position during this time ?
The camera shutter is closed then, during the extra noise processing to examine the dark space. So moving the camera then will not affect the picture, the shutter is closed, and the Viewfinder is restored functional.
The dark image for noise reduction is taken under the same conditions as the original image and the images are subtracted. Long exposure noise is highly dependent on sensor temperature, so if you move the camera in a manner where it is exposed to different temperatures, this can affect the result. The shutter remains closed so image shake or its ilk is not much of an issue, and repositioning while on a tripod should be pretty harmless in most cases as well.
Moving from sunlight to shade and vice versa is likely a bad idea in that time span. And in freezing environments, keeping the temperature situation similar for shot and aftermath is likely a good idea: if the camera has been sitting outside for several minutes before hitting the trigger, putting it back into the warm coat should likely wait until the NR compensation image has been completed.
