NSA

[P3D Kenneth Luker <kluker@xxxxxxxxxxxxxxxxxxxxxxx>]

I haven't made the time since NSA to write up a complete report, but I
should at least give a quick report of what I saw as the highlights, so
that I can record them before my brain dumps the info.

I haven't yet seen a review or summary of Bob Brackett's workshop on 3 D
using a single camera.  I almost didn't go, because I shoot almost
exclusively in Realist format (although that is a single camera, that's
not what he meant.)  But I'm glad I did.

Bob's mind must be continually inventing things!  He showed us a gizmo
that is a combination slide bar and viewfinder.  I understood that he
won't be making them for sale, because he had an even better idea later.
 But I get ahead of myself.  This gizmo had a camera mount that would
slide from side to side of the slide bar, and the slide bar had a
built-in view finder.  The finder in the slide bar made it possible to
keep the device at the eye while shooting, so the aim-point and framing
would remain the same, eliminating toe-in, rotational error, etc.  The
device could be tripod mounted, I believe, but the model he showed had a
handle for holding it at eye level by hand.

The innovation, in addition to the finder-on-the-bar was that the mount
itself was spring loaded.  He could force the camera to one side against
the compression of the spring, and as he shot the picture, the spring
would move the camera immediately into position for the second shot, and
he could squeeze that one off without taking his eye from the finder.  He
recognized the need to prepare oneself for the change in balance of the
device as the camera shifted, the need for motorized film advance, and
the need for being aware of vibration (hold it steady for the second
shot).  The time that elapsed between shots was short enough that most
views without rapid movement could be photographed (about one second).

The innovation that he thought would make the slide device unnecessary
(at least with respect to the function of allowing sequential photography
without the eye leaving the viewfinder) was his observation that
regardless of what lens is being used, within the viewfinder of a
single-lens reflex camera the appearance of the sight-picture is
unchanging, usually consisting of some bracket marks, a split-ring
range-finder image, etc.

Bob noted that one of the main drawbacks to cha-cha photos (weight
shifting from one foot to the other to get the inter-ocular spacing for
stereo views) is that the amount of shift necessary to permit proper
projection of the resulting pictures is difficult to calculate, and many
pictures have hyper-stereo that is uncomfortable, or may have on-screen
deviations that are painful to view.  He revealed a method, in the
workshop, that will allow anyone to do cha-cha photos with the assurance
that the resulting pictures will be projectable (will have on-screen
deviation that allows the near-point to be at the window and the infinity
point to have 2.5 inch deviation maximum),  and they can take them
without removing their eye from the viewfinder.

Bob produced a 2.5 inch black disk with stick-em on the back so that he
could plant it directly in the center of the lenticular screen where it
would stick.  He then stood in the middle of the room with his SLR camera
at his eye and moved to a point where the screen just exactly filled the
view finder.  Then he noted mentally the location, size and placement of
the image of that black disk within the viewfinder.  By memorizing that
picture, he could remember the size of the disk, which showed him how
much space within the viewfinder the maximum on-screen deviation must
take when he was shooting cha-cha pictures.  Thus, when taking a picture,
he could note within the viewfinder the placement of the nearest object.
 Then, making sure that the most distant object (or infinity) fell at
exactly the same place in the frame of both pictures, he could cha-cha
just far enough for the nearest object's image in the viewfinder to move
exactly the distance that represented the width of the 2.5 inch disk.
The pictures taken in this way could be mounted so that with the nearest
object at the stereo window, the most distant object would fall precisely
at 2.5 inches separation on the projection screen.  Bob's insight was
that this is true regardless of taking-lens focal length, projection-lens
focal length, wide angle, telephoto, macro lens, whatever.  Even
convergence (or toe-in) is automatically accounted for and allowed, if
needed to conserve frame space.  Bob explained this several ways, but one
of the most visual was his characterization of the goal as being to use
the maximum amount of stereo space allowable in a stereo pair, every
time.  That maximization will occur when the near-points (or window
placement) coincide spatially on the screen (so the window or near-point
is at the screen) and the far points are at 2.5 inch separation (or
normal eye separation).

He acknowledged that this works only if you have taken your sight-picture
with the black disk placed on the exact screen you will be projecting
on--that is, on a screen of the same size you will be using to project
onto.  Otherwise there may be problems.  If you project at home on a 40 x
40 screen and at the club on a 70 x 70 screen, the method won't let your
pictures be shown perfectly both places.  If you want to shoot for the 70
x 70, go put your disk on that screen and memorize the sight-picture
there.  Then shoot your cha-cha while remembering the appearance of the
black disk on THAT screen.  (The disk will appear smaller within the
large screen, proportionally, so the amount of shift allowed will be
smaller).

He suggested that one way to compensate for this problem would be to set
up the projector at the club so that the window was really at the
distance of a 40 x 40 screen, but focus on the more distant large screen.
The window would fall in front of the large screen (the frame edges
wouldn't coincide) but the infinity points would be within the 2.5 inch
maximum.  This would be a compromise to allow projection on either
screen.

All this seemed obvious, once Bob pointed it out.  I don't know if anyone
had an intuitive understanding of the concept before his explanation,
though.  Maybe that's the nature of invention.

I'll post more about NSA later, but I wanted to get that one out to the
public.

Ken Luker



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