51,200 is the maximum Iso on the D750, being Hi2. I have gotten images at 51,200 when absolutely necessary, but have found 20,000 to be around the limits for me in general. Worlds away from my days with the original 5D, Minolta 7D or the D70, but I still look forward to improvement. Another 2 stops would let me shoot bands that are nearly off limits currently.