The pop-up flash on the D800 has a respectable guide number of 12m at ISO 100. Flash exposure compensation can be set from -3 EV to +1 EV.
Of course, much more advanced options - including wireless control - exist when using one (or more) of Nikon's optional Speedlight external flash units.
The issue of lens diffraction at smaller apertures has long been a concern for photographers looking to eke every last ounce of detail out of their images. And with the D800 sporting a 36MP sensor, image quality distinctions can be evaluated at a level that was nearly unthinkable just a couple of years ago. We thought it would be interesting to examine the performance of the well-regarded Nikon AF-S Nikkor 24-70mm f/2.8G ED lens at a range of apertures in JPEG mode. In the series of images below, the camera was mounted on a tripod and the lens was set to a 70mm focal length at ISO 200. The crops show 100% views of the area highlighted in red in the top image.
Look carefully enough and you can probably see a subtle difference in sharpness between the images shot at f/4 and f/5.6 compared to the one at f/8. By f/11, the effects of lens diffraction start to become more pronounced and at f/16 and f/22, a noticeable amount of sharpness is lost (but as you can see from the following samples, careful sharpening of a raw file can still yield a decent result).
f/22 (100%) JPEG
These crops are taken from the same file that we've shown above, in JPEG mode at f/22. As you can see, at a pixel level, there's very little fine detail and sharpness is relatively low.
f/22 (100%) RAW (converted and sharpened 'to taste')
A few minutes spent adjusting Photoshop ACR's sharpness sliders though and a lot more detail can be drawn out of the simultaneously-captured .NEF raw file.
Its important to keep in mind though that the above examples were shot in well-lit conditions at a low ISO. By ISO 1600, chroma noise and the resulting in-camera noise suppression algorithms - while by no means objectionable - are the more pressing factors limiting image sharpness and detail. We're not saying that you shouldn't care about diffraction, but simply that for day to day photography you probably don't need to worry about it as much as you might think.
Also not to be overlooked is the very real possibility that the benefits of increased depth of field can many times outweigh diffraction-induced softness. Below you see a second pair of crops from the files shot at f/4 and f/16 respectively, showing foreground foliage. The increased depth of field you get by shooting at f/16 is significant, and may well be more important in a print than the slight diffraction-softening of the distant buildings.