Some Nikonos lenses only work under water?

[Emil]

A lot of websites about the Nikonos cameras and lenses state that some of the lenses will only work under water. Does anyone know why that is? And shouldn't it be possible to move the lens further from or closer to the film to get it to focus in air?

 

[nyoung]

IIRC some of these lenses have the focus calibrated to optimize performance with different diffraction of water as opposed to air. You probably could build an adapter to accomplish what you are imagining but given the prices of lenses today, it would probably not be cost effective

 

[Pinholemaster]

The 35 mm can be used in air and water.

The other wider lenses are engineered to use the water was the first element in the lens configuration. Perhaps if you add a new front element you'll be able to use these lenses topside, but why go through the hassle.

 

[ragc]

The 35mm f2.5 and the 80mm f4.0 have flat plate glass ports. They are meant for use in both air and under water. All others have domed ports. Refraction at the glass surface of the lens' port bends the light waves by an approximate factor of 1/3 (I'm going from memory here, it could be 1/4!). Because of this, distances under water also appear 1/3 shorter (objects appear 1/3 closer). This can be clearly seen if you dive with a diving mask on. If you remove the mask no refraction occurs and distances are correct.

Under water an image forms at the interface of the port surface with the water. Domed ports preserve the angle of the lens' air optics by stretching and bending the image along the curved surface of the glass, so a 15mm underwater lens is still seeing a 15mm field of view. Domed port lenses are designed to focus on the curved image on the domed port.

A lens with a flat port will not compensate for the refraction index of water, and the angle of view will be narrowed at the glass/water interface. The 35mm's focal length will thus will be "increased" to 46.55 approximately (1/3 longer), but the image will be flat. The lens is designed for the flat image, as on dry land.

Flat port lenses work equally well on land, as the flat glass is just another "filter". Domed port lenses will distort out of water because they are designed for the curved image, but in air the domed port behaves just like a flat port. Thus the optics of the lens, which were designed for the curved image, will distort the image if used in the air.

The 28mm underwater lens has such a slightly curved port it can be used in the air, but it does distort a bit.

 

[tony lockerbie]

What ragc says is quite correct. I find the 28mm is really unsatisfactory as an above water lens, the 35mm and the 80mm are the only real dual purpose lenses. If you are contemplating below the water use, the 28mm is perfect, but the 35 is not too bad either.