T3D Re: Underwater stereo photography

[Bill Costa -- Network Info Srvs <bill.costa@xxxxxxx>]

Folks!

    The inimitable Dr. T  --- a.k.a. fj834@xxxxxxxxxxxxxxxxxxxxx 
    (Dr. George A. Themelis) -- recently posted to tech-3d:

> While still on this subject, can someone explain why small
> stereo bases are used for underwater photography?  Why
> is RBT marketing a RBT-S1 with 45 mm separation which I am
> told is bought basically for underwater photography?

    I had the same question myself, so I did an experiment and found
    that it is probably because of the magnification effect of the
    air/water interface.  Most divers are aware that when they are
    underwater, objects appear magnified.  What I found interesting
    was doing some tests in a pool and finding that the effect is even
    greater than I realized.  First here's an analogy...

    I find it interesting how we humans pretty much ignore the color
    balance of light.  A room lit with incandescent bulbs looks fine
    to our eyes, or maybe a little warm.  But not at all the way it
    looks when we take a photo under that light -- positively reddish!

    Well I knew intellectually that my view underwater as a diver with
    a flat glass mask was magnified.  But it wasn't until I did some
    actual photograph tests that I came to appreciate the amount of
    magnification.  According to 

            http://www.cyber-sea.com/htmfiles/art3.htm

        "Divers learn in their first open-water training classes that
        objects underwater look 25 percent closer and 33 percent
        larger when viewed through a standard mask, equipped with
        'flat optics'."

    I'm not sure I follow why it's 25% of one and 33% the other, but
    no matter.  What I did was create a photographic target out of PVC
    pipes and joints.  This rig held up 4 plastic plates at fixed set
    of distances from the camera.  The rig also provided a tripod
    thread at the same level as the plates so that the camera would
    always be exactly the same distance from the targets.   It looked
    a little like this:

                            O
                            |          O
           []=         O    |     O    |
           |           |    |     |    |
           |           |   /      |    |
           +--------\--|--/----\--|----+
                      \|         \|


    Comparing the poolside and underwater photos was quite a
    revelation.  I never appreciated the degree of magnification
    involved.

    In any case, I used a slide bar to take photos at the two offsets
    of the S1a and S1b.  I also took a lot of above water photos at
    those two same offsets of just general scenes -- covered bridges,
    scenic vistas, etc. etc.  What I was trying to do was determine
    which camera I should buy -- Could I use an S1a (59mm offset) in
    the water?  Would I be happy with an S1b (45mm) on dry land?  (And
    worse still, the German housing has to be custom made for one or
    the other.  No starting out with one and then switching later.)

    My conclusion, alas, was that I want one of each!  But if I *had*
    to pick one, it would be the S1b.  On land there was not a *big*
    difference between the two offsets.  But underwater there was.
    The 45mm offset looked natural and was easy to fuse.  The 59mm was
    do-able, but really was too much to be comfortable.  And that was
    in a Red Button with an adjustable interocular.  I imagine
    projection would be worse.

    BTW -- the camera used was a Nikonos with the same focal length
    lens as the S1 (35mm if I my memory can be trusted).  And when I
    say a Nikonos -- I mean just that.  Not a II, III, IV, etc.  This
    was the first model with that name.  And just like an old stereo
    camera, I have trouble with some of the shutter speeds and with
    occasional overlapping on the frames.  :-)   BTW, in my jig, the
    closest target was at 0.6m, the min focus on an S1.

> Is a Realist (70 mm separation) not a good choice?  

    Based on my experiments, I'd say not!  (Plus see below.)

> Would an
> ISO Duplex (30 mm) be better?
> 
> How about focal lengths?  Is 35 mm short enough for UW photography?

    With underwater photography, the shorter the focal length, the
    better!  But perhaps not for the reasons you would first think.  A
    35mm is OK, but a 28mm is preferred.  And the lens that most
    Nikonos owners lust after?  The 15mm!  (A very expensive piece of
    glass -- and this for a lens that only works underwater!)  Why so
    short?  Visibility is always a problem in the water.  Even in the
    gin-clear waters of the Caribbean, you want to have as little
    water between your lens and your subject as possible.  More water
    means more particles which means more backscatter from the flash,
    etc.

    Probably more than you wanted to know on this subject, but...


                                                        Later....BC


    PS: I got a phone call from Lynn Butler, a published photographer
        who recently asked me these same questions.  Jon Golden sent
        her to me; seems that she too wants to do stereo above and
        below water with an S1 but was having trouble deciding which
        offset to get.  Seems even professional photographers who are
        use to using Nikons, 'blads, and Leicas find RBTs a bit 
        pricy!  :-)
 

-- 
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| Bill Costa                                          |
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