D800 and 12-24mm lens

What gives best imagequality?

D800 in dx mode at 12mm

Or

D800 in fx mode at 18mm

If you are talking about the Nikon 12-24mm/f4 AF-S lens, which is a DX lens, I wouldn't use it on the D800. The image circle of that DX lens may appear to cover the entire FX frame, edge and especially corner image quality is very poor. My experience is that the 12-24 is good on FX only at 24mm.

 

Even though you use DX crop on the D800, and it becomes a ~15MP DX body, edge quality at 12mm is still mediocre.

D800 in DX mode with the 12-24 set to 12mm

 

Here's an older thread on the same subject: D800 with Nikkor 12-24 f4 Mounted?

It's usable as a full frame lens around 18mm, but as others have said the edge sharpness is very poor.

 

Do not go below 18mm or you will see edge distortion like you have never seen before. The lens will fill the frame at around 14mm, but only just.

 

I agree with others, too, on the general sharpness of this lens. When I only had 12mp DX DSLRs, I was happy with it. Actually, now that I still only have 12mp DX DSLRs, I remain happy with it on those bodies :) . It starts to fall apart if you increase resolution. The newer 10-24 is a MUCH sharper lens, and the old 12-24 is cheap for a reason.

the old 12-24 is cheap for a reason

Cheap? Still being sold new for $1147 at B&H. Even used it still costs between $350 and $550 (unless in beat up condition) - more than some better lenses can be had for new.

For FX usage, I prefer the newer 18-35mm AF-S, which is not all that expensive: AF-S NIKKOR 18-35mm f/3.5-4.5G ED | NIKKOR D-SLR interchangeable lens from Nikon

 

I also have the older 17-35mm/f2.8 AF-S and 14-24mm/f2.8 AF-S, but usually I prefer the 18-35mm for my wide-angle usage, unless I need really wide 14mm or indoors at f2.8.

Cheap? Still being sold new for $1147 at B&H. Even used it still costs between $350 and $550 (unless in beat up condition) - more than some better lenses can be had for new.

 

Okay, maybe "cheap" is a bit off, but on the used market it's a fair bit less expensive than the 10-24.

 

I'd have to go back and check, but I think I paid $350ish for my KEH Bargain example. A comparable 10-24 was about $200 more when I bought it.

I paid US$900 for my 12-24mm/f4 DX AF-S (new lens) back in 2003 or so, when it was the first DX lens (and first DSLR lens dedicated to the APS-C format). My camera at the time was a 6MP D100. I still own that 12-24mm lens.

Purchased mine used in 2005 for $800 - and traded it in (a bit heavy-hearted) together with the 80-200/2.8 in 2011 for the 70-200/2.8 VR.

 

KEH Bargain example

those are below $300 nowadays.