Originally posted by Marc Sabatella I don't know, but if I wanted to find out, it seems it would be quite simple to test that for oneself - take pictures at ISO 800 with the threshold set to 400, 800, and 1600, then pixel peep and decide if the one taken with the threshold at 800 looks more like one with the threshold of 400 or 1600.
No need to pixel-peep and guess, the EXIF contains the answer. I just took a series of pictures from ISO640 to 1600 in 1/3EV steps, with "NR Start Level" set to "1. ISO800".
ISO : 640
High ISO Noise Reduction : Unknown (2 0 56)
ISO : 800
High ISO Noise Reduction : Unknown (2 0 56)
ISO : 1000
High ISO Noise Reduction : Unknown (2 1 56)
ISO : 1250
High ISO Noise Reduction : Unknown (2 1 56)
ISO : 1600
High ISO Noise Reduction : Unknown (2 1 56)
Note the middle number going from 0 to 1 at ISO1000, not 800. From that, I conclude that the text accompanying the "High ISO NR Start Level" option is correct, i.e. NR kicks in at an ISO *greater than* the one in the chosen option.