Here is a quick comparison of ISO performance (low ISO and high ISO) between the Nikon D600, Nikon D700, Nikon D800E and Nikon D3s. Please note that all of the images below were shot in JPEG, since Nikon D600 RAW support is not available yet. All images were also down-sampled to the Nikon D700/D3s resolution (cameras with the lowest resolution). Everything was shot in ambient light (lab results are posted in the Nikon D600 review here) with all camera corrections turned off and camera profile set to standard (default, no changes). Cropping and export was performed in Lightroom 4 and I used Photoshop to add the text on the bottom of each image.
1) Nikon D600 ISO Performance
2) Nikon D700 ISO Performance
3) Nikon D800E ISO Performance
4) Nikon D3S ISO Performance
Quick analysis: very impressive performance by the Nikon D600. I do not see any difference in performance between the Nikon D600 and the Nikon D3S all the way to ISO 6400. Boosted ISO levels on the D3s above ISO 12800 look better, but that’s expected. Nikon D700 is a thing of the past when compared to any of the above cameras. The clear winner here is the Nikon D800E. Not only because it has the most amount of details (thanks to down-sampling), but also because the noise it produces looks very natural. Again, note that this is a preliminary analysis using JPEG output only (which I would not rely on, since JPEG rendering is not the same as RAW conversion). The above test will be redone using RAW files when Adobe releases NEF support for the D600.