That's good. One of the keys to focal length and a broad depth of field is the hyperfocal distance. Focusing your camera at the hyperfocal distance ensures maximum sharpness from half this distance all the way to infinity, the hyperfocal distance varies with the aperture (f-stop) used and the focal length of the lens, and really demonstrates the difference between using wide angle and telephoto lenses. The web-site below includes a hyperfocal distance calculator I would recommend you try out.
http://www.cambridgeincolour.com/tutorials...al-distance.htm
Your picture was taken with a 50mm (f1.4 min) lens at F5.6 and 1\320 at ISO 2500 with a Nikon D3 (Full frame DSLR - very nice kit by the way...)
For a "standard:DSLR with a 1.6X crop factor (or using DX lens on a full frame nikon) the hyperfocal distance for a variety of lens at f5.6 and f16 is
Focal Length f5.6 f16
16mm 2.2m 0.8m
24mm 5.0m 0.9m
35mm 10.6m 1.9m
50mm 21.7m 3.8m
85mm 62.6m 11m
135mm 158m 27.6
200mm 346.7m 60.7
Your camera is full frame, so as long as you are not using a DX lens you need to change the camera type in the calulator to 35mm. So for your camera using
Focal Length f5.6 f16
16mm 1.4m 0.5m
24mm 3.1m 1.1m
35mm 6.6m 2.3m
50mm 13.5m 4.7m
85mm 39.1m 13.7m
135mm 98.6m 34.5
200mm 216.5m 75.8
So using the settings you used, (50mm at f5.6) your hyperfocal distance would be about 13.5m.... So for everything to be completely in focus the end of the lens would need to be half that distance away from the closest object you want in focus 6.2m (and you couldn't get that closeup look you have at that distance). Stopped down to f16, you would have considerably increased your depth of field, but would have needed to reduce your shutter speed to about 1/90 second (and could have gotten movement blur if the animals didn't co-operate) if you were less than 2.4 meters away the background would not have been completely sharp (but I personally wouldn't want it that way anyway).
I would have been happy with the sharpness of shot you have (at least in the resolution I see on the post). For the iso (2500) there is very little noise (makes me jealous and really want to switch from my canon 1Dmk3 to one of the new nikon cameras). The fact that you have a full frame camera and great noise reduction (nikon d3) means you really do have the best equipment for these kind of shots. A slightly wider lens would make your job easier. For most portrait work, the part that is really important to be in focus is the eyes - so really check for this when you are framing the shot. Dogs can be more of a challenge to photograph than humans as more depth of field is required to get the nose and eyes in focus (particularly head on).
If you have stationery objects in low light adjusting the depth of field by increasing the shutter speed is relatively simple using a tripod to stop camera shake. With moving objects in low light, and no flash lighting there are optical limits to what you can achieve.
Anne