I must admit to wondering where the Nikon Aperture values have been quoted from.
The often quoted rule of thumb for a diffraction limited print size of only 10"x8" is f20 full frame and f16 crop. Using a smaller apertures will visibly degrade the quality of a 10"x8" print.
To produce a critically sharp A3 print you need considerably bigger apertures (couldn't find the exact values with a quick Google). My personal rule of thumb is that unless I have a creative reason for doing otherwise (like needing more DoF in a landscape or less DoF in a portrait) I'll use f5.6 to f8.
Here's an interesting chart.
http://www.the-digital-picture.com/Reviews/Canon-EOS-50D-Digital-SLR-Camera-Review.aspx
This shows the aperture needed to get sharpness at the pixel level. The kind of biting sharpness that you get when you think the zoom button has broken because it won't take you in far enough.
I believe your D800 has similar pixel density to the 60D - hence to get an image that looks sharp at the pixel level you need to be using f6.9, anything smaller will be blurring the pixels. :|
Cue people pointing out that aiming to get images sharp right down to the pixel level is a pointless exercise; they have a point. But hey, it's a D800 and its purpose in life is huge pixel count.
However, taking critically sharp images is hard to beat as a benchmark for improving camera technique. EVERYTHING affects sharpness, not just aperture. You need perfect technique including decent glass, cable release (or self timer), tripod, mirror lock up (or live view), base ISO and perfect exposure. The slightest mistake will show.
Once you are confident you CAN take critically sharp images, you know the compromises you are making when taking 'normal' shots and how significant those compromises are.
If you are only taking to print at A3, you can get away with murder - even though you are wasting all those lush pixels.
As for solving the depth of field problem in landscapes....
I invested in a Tilt Shift lens - From experience I know that half a degree of down tilt allows me to take a standing hand held image, portrait format with the horizon on the top third and everything will be critically sharp from my feet to the horizon, even at f3.5. Aperture is only used to give depth to the plane of focus! It probably made a bigger difference to image quality in my landscape images than upgrading to the 5DII.
Do I get a prize for the most rambling response of the day
) is that sharpness is primarily about pixels when in practise it's much more about lenses and image contrast. Not to mention the techniques essential to optimise image quality. For example, when you see a nice crisp image on your computer screen, just how many dots are you seeing? It will be between two and three million max, for an A4 image.
I much prefer the output from the 5D than the 60D.
