For basic information - 14mm f2.8 fixed focal length lenses for Nikon cameras have been made by both Sigma and Nikon. And also, I believe by Tamron. Canon has also made several versions of a 14mm f2.8 lens. ALL of these 14mm lenses were made originally for full-frame (FX in Nikon terminology) cameras, and in most cases made for film cameras prior to the existence of full-frame digital cameras.
I have been a fan of this focal length lens as an excellent lens for over/under shots. I posted my first "pool test of and over/under shot with the 14-24mm lens some months ago as well as several much older over/under shots taken with Sigma's previous 14mm f3.5.
There have been some significant differences between the 14mm lenses I have owned, to which I recently added Nikon's 14-24mm f2.8. The Sigma 14mm f2.8 lens used their HSM focusing, Sigma's equivalent to Nikon's AFS. This made it very easy to switch between manually focusing and auto-focusing the lens when you wanted to do this. This (AFS or HSM) is very useful if you have a housing, like my Seacams, which make switching between manual and autofocus VERY DIFFICULT to IMPOSSIBLE when you are at any significant depth. The Sigma 14mm needs only a single manual focusing gear on the lens to accomplish this with ease. To the best of my knowledge, Sigma no longer makes their 14mm f2.8 lens. When it was available, the Sigma 14mm f2.8 could be bought for about US$600.
The Nikon 14mm f2.8 IS NOT AFS and has a manual-autofocus selector switch similar to the one on older models of Nikon's 60mm and 105mm Micro-Nikkors. I was able to build a complex pair of gears to do this with the Seacam housing. My opinion, based on my single Sigma 14mm lens, and my single Nikon 14mm lens, is that the Nikon 14mm lens has slightly better edge sharpness and better flare characteristics when the sun is in the frame, or just beyond it. Having a lens shade on your dome port that is made specifically for use with the 14mm can make a big difference in flare control.
While I haven't actually shot comparison shots, at the same place and same time, I believe the new 14-24mm lens is as sharp at 14mm as the fixed focal length 14mm Nikkor. 14-24mm shots with the sun actually in the picture seem to show more of the bright little "sunspots" than does the 14mm fixed focal length lens. This may be due to the fact that the zoom has more elements and larger front elements. I like the versatility of the 14-24mm zoom and feel it is a good back-up to my 17-35mm f2.8 for general underwater shooting in addition to giving me the 14mm wide end for over/under shooting. Overall, however, I feel the 17-35mm lens is more versatile for general wide shooting and would carry only that lens if I had to travel very light.
At some point in the near future, I will probably sell both my Nikon and Sigma 14mm f2.8 lenses along with gears for use with Seacam housings. Anyone interested?
Fred