I do not see any argument between the sharpness of a fixed focal length prime lens versus a zoom lens, the fixed focal will be sharper.
What is your D800 experience with diffraction and macro shots? Does diffraction obliterate the gain in depth of field with small f stops?
I haven't noticed degradation at f22 underwater, although it's obvious in carefully taken images on land.
Nudibranch, Seraya Secrets, Tulamben: Nikon 105mm, 1/250 @ f22, ISO 200, Inon D2000 strobes x2, 30cm from focal plane
My personal dislike of the zoom lens is its relatively poor performance at longer focal lengths (wasting so much of the D800's capability) and an aperture that's just not wide enough for underwater shooting with the smaller strobes that are useful in macrophotography.
Edited by tdpriest, 21 June 2013 - 04:11 PM.