MTF/Distortion on 20mm/2.8-UW & 15mm/2.8-UW

[joseo]

Dear all,
I am looking for information on the Underwater lenses 15mm/2.8 and 20mm/2.8 for Nikonos. Does somebody have MTF chart or/and distortion information? Somebody told me the lenses are very sharp compare to other in air lenses, and that is needed UW. That trigger my curiosity. I am also going to do some measurement underwater, hence I need this information.

Best regards
Jose Åsheim Ojeda

[Bjørn Rørslett]

I have both lenses and have used them on various Nikonos cameras. They both are very sharp lenses, and share the need for being fully in contact with water to form a sharp image. Out of water they cannot focus at all.

The 15 has a slight barrel distortion at its closest distance setting. The 20 slightly less, but at the same time it does not focus equally close.

[joseo]

Thanks for the information. Do you have any experience of using them under water? At what working distance are they best suited? Is the wide one getting too wide at say above 1-1.5m working distance. My objects are fishes with typical 60cm length, I want to measure them at 1-1.5m focus distance. Illumination will be IR during night, else none. I know it is very much absorbed, but tests show I will manage 1.2-1.5m working distance with my current ilumination. Will the IR focus fall very far away from the visible if focused at 1m for these two lenses (maybe they are AR coated to the VIS only) sa used at f/4?
 
Do you have any ideas on where to find MTF and distortion charts? Maybe through Nikon in Norway?

Thanks in advance
Jose

[Bjørn Rørslett]

These lenses can only be used under water.

They focus to infinity of course, but underwater scenes are best shot quite close due to the rapid light attenuation at distance. Thus 2-3 m away is about the maximum distance, unless one works in very clear water.

If you wish to compute the angle of view,  the refractive properties of water have to be taken into account. Thus compare them against 20 and 28 mm lenses for the FX format. Should be easy to find with a net search if you don't have such lenses available yourself.

I do have used the 15 Nikkor with IR (actually, Infrared Ektachrome) and distinctly remember how severely IR was depleted. Cannot recall any focus shift, although it is bound to occur.

As these lenses were designed and built long before the digital age, no MTF or distortion charts are likely to exist.

[paul hofseth]

I am examining old lens posts and find both interesting digressions and themes such as this thread on page 41,  Here is a topical digression supplementing the practical advice above:

Not just an issue of the passage of time since the UW Nikonos was designed, but a physical stumbling block.

Numerical clarification of this particular thread wil call for waterproof measuring devices (an undrwater zeiss K-8?) unless one finds a gas which under high enough pressure will have the refractive properties of water.  Or a well lit water tank enclosing lens front and target will be ncessary in order to measure MTF for UW optics. An expensive undertaking -.

p.