Canon EOS-1D X Mark III EOS-1DX Mark III Advanced User Guide - Page 181

Long Exposure Noise Reduction Noise that tends to occur in long exposures at shutter speeds of one sec. or slower can be reduced. 1. Select [ : Long exp. noise reduction]. 2. Set a reduction option. Auto For images exposed for 1 sec. or longer, noise reduction is performed automatically if noise typical of long exposures is detected. This setting is effective enough in most cases. Enable Noise reduction is performed for all images exposed for 1 sec. or longer. The [Enable] setting may reduce noise that cannot be detected with the [Auto] setting. 181