Nikonos lenses only focus underwater?

Discussion in 'Nikon' started by Paul Furman, Nov 12, 2007.

  1. Paul Furman

    Paul Furman Guest

    What does that mean that some nikonos lenses only focus underwater? I
    did a little searching and the only mention of incompatibility on dry
    land is that some lenses such as the 15mm f/2.8 have really bad flare
    above water which is not an issue underwater.

    I was looking at a water damaged 15mm f/2.8 UW-Nikkor to tinker with and
    I guess because of the focus distance it's probably hopeless but I also
    read that version had some redesign to correct some backfocus issue so
    perhaps it's possible to use on a DSLR by removing the nikonos mount and
    rigging my own mount. It's a rangefinder type system with no SLR mirror
    so presumably a closer distance from the rear element to the film/sensor
    but maybe not with the backfocus thing and the smaller mirror on a DSLR.

    Back to the underwater focus question:
    <http://www.mir.com.my/rb/photography/companies/nikon/nikkoresources/ultrawides/15mm.htm>
    "UW Nikkor 15mm F2.8 Lens with Viewfinder Nikonos 15. This UW Nikkor
    15mmhas a angle of view corresponds to a 20mm land lens, providing a 94
    degree view."

    hmmm....
     
    Paul Furman, Nov 12, 2007
    #1
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  2. Paul Furman

    Peter Irwin Guest

    Water has a higher refractive index than air.
    The wide angle UW-nikkors (except the 35) have
    curved front elements which are directly exposed
    to the water. The design of the lens is based on
    having water in front.

    The 35mm lens has a flat piece of glass in the front
    and is designed to have air between the front element
    and the protective glass. It will work both underwater
    and in air. The optical design of the shorter
    lenses depends on having a medium in front with a
    refractive index of about 1.33 (water) rather than 1.0 (air).
    Since the front of your 15mm lens is strongly convex,
    it should have an even shorter focal length when used
    above water.

    Peter.
     
    Peter Irwin, Nov 12, 2007
    #2
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  3. Paul Furman

    Peter Irwin Guest

    There is some useful information here:

    <http://www.nikon.co.jp/main/eng/portfolio/about/history/nikkor/n08_e.htm>

    Peter.
     
    Peter Irwin, Nov 12, 2007
    #3
  4. Paul Furman

    Paul Furman Guest

    I thought I saw one web page which showed the 15mm f/2.8 without it's
    waterproof container, but probably the outside element (which is curved)
    is part of the optical formula.
    Thanks, apparently though, it works the other way around, it *is* a 15mm
    lens but has the field of view of a 20mm lens underwater. Because there
    is no rangefinder mechanism or mirror for TTL focusing, it is focused by
    guess or tape measure with a DOF chart. So that distance reading on the
    lens would be wrong on dry land but usable with a conversion chart
    presumably. And, I assume it has a closer rear element to film/sensor
    distance so it wouldn't be possible to adapt to a DSLR though I've not
    actually found anything to confirm that.
     
    Paul Furman, Nov 12, 2007
    #4
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