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Are they specially constructed for underwater use only? I am looking at

http://cgi.ebay.com.au/ws/eBayISAPI.dll ... NARL:AU:12

Yes they are Justin they fit the Nikonos range of underwater cameras, speciall constructed so they need no underwater housing. Wouldn't know if they can fit on a DSLR, would assume not as I would imagine there would be some seals, etc. Some lenses work only underwater, some both, I don't know if that is due to diffraction of the water or something else

Link:
http://www.nikonimaging.com/global/prod ... /index.htm

Ahh - anyway it's an f4 not f1.4, the ad is slightly misleading....

Just for reference:

UW-Nikkor 20mm f/2.8
Great for up-close shooting, to 40cm (1.3ft.). Underwater only.

UW-Nikkor 28mm f/3.5
Standard wideangle lens. Underwater only.

W-Nikkor 35mm f/2.5
General-purpose amphibious lens.

Nikkor 80mm f/4

So the 80 can be used normally, should you wish to!

Yeah, those lenses are to be used underwater only.

From what I know, they are made of pressure resistant metal sealings. So are heavy stuffs. The mount should be different to normal F-Mounts which will have a special seal lock that regular mounts do not.

Edit:
as for my rough guess on "underwater only" is that light refraction index of water and air is different. So as the optics in the front elements (or every glass) are corrected to transmit light under those situations. And thus reduce the cyan/blue tint under sea water, as so boost up other underwater objet's saturation/contrats....
Just my rough guess, dont take it too serious tho