High ISO images are impressive for both cameras. (Obviously the D4 does better than the D800 as it should). What is most impressive perhaps are the base ISO images for the D800 - WOW! There doesn't seem to be much of an improvement in IQ at the extreme (high) ISOs when comparing the D3S to the D4, especially above ISO 6400, although considering you have a higher pixel count, that is an improvement in itself. Another pleasant surprise as expected - the D800 seems to do a bit better at high ISO compared to the D700. FWIW: I like to use the test image at the very bottom of the groups as it has a variety of interesting areas of detail and the entire ISO range. Here is a link the the Comparometer at the Imaging Resource site: http://www.imaging-resource.com/IMCOMP/COMPS01.HTM For those of you not familiar with the site, once you select the camera body, a series of test images will appear. Click on the image once to bring up a small view of it. Click on the image a 2nd time to see it at 100%. Bring up one image on the left and a 2nd on the right and you can compare them.