In anticipation of an upcoming trip, I decided to acquire another Nikon full-frame sensor DSLR. This one will supplement and not replace the D700.
While the D800 was recently released by Nikon and could be considered to be a D700 successor, the more recently released D600 seems like a closer relative to the D700. In a nutshell, the primary features of the D600 include a 24-megapixel sensor (vs. the 12-megapixel sensor of the D700), video recording capability, comparable low-light performance to the D700, and a lower introductory price than the D700.
Unfortunately, the changes are substantial relative to the D700 - different type of batteries, different remote trigger, different AC power interface (not available yet), and different controls and software. From a practical standpoint, while the user interface is quite different, the functions are similar, so it's not that tough to move from the D700 to the D600. The real problem I'm anticipating is confusion between the two in the dark of night with a brain fogged from lack of sleep.
Because low-light performance is my primary concern, the first thing I did was to run a couple of quick qualitative comparison shots which are presented here. Remember that these are shots from the particular units I happened to get and may not be representative of other cameras of the same model.
The comparison shots at right (click on the thumbnail for a full-sized image) were done at ISO 6400 to ISO 200 with both cameras set to aperture priority and the same lens used (35-70mm zoom) at 35mm, f/22. All shots were done in raw mode. The automatically determined shutter speed is shown below each frame. Just the central part of each frame is shown in order to save space. For my purposes, the D600 result is close to the D700 performance, with the edge going to the D600, perhaps due to the finer pixel pitch, but at the expense of files which are twice the size. At any rate, my fear that low-light performance was sacrificed to squeeze more pixels into the D600 seems to be unfounded.
And finally for the sake of completeness, 5-minute exposures (ISO 1600) done in total darkness are shown at left. As in the previous set of images, both cameras were shooting in raw mode. These were brightened equally in Lightroom 4.2 (exposure setting at +3.0) The full frames are shown at the top of the image, which reveal a slight pattern in the dark frame signal. Below each full frame are enlargements of the central portions.
Note: the D700 includes a built-in diaphragm over the eyepiece port which prevents stray light from leaking into the image. The D600 does not include this, so an external eyepiece cover is necessary for long exposures.