Rangefinders and Bill Ewald

[T3D <BD3D@xxxxxxxxxx>]

Hi all.  I took the liberty of making a copy of all the recent posts about
rangefinders, ranging binoculars and parallax bars, then dropped it off at
Bill Ewald's when I took my daughter Diana over for Trick-or-Treat.  Knowing
of Bill's interest and expertise on the subject of rangefinders, I thought
he might find the discussion interesting. Sunday Bill arrived at my house
with six pages of handwritten responses.  "Was it interesting?" I asked.
"Enormously!" was his reply.  Here are his comments:

"The Internet info makes me realize what I am missing.  I can give you a
little follow-up with my lead pencil communicator.

"Re: Peter Abrahams.  I was a very close friend and associate with Hubert
Nerwin who had been an engineer & camera designer for Carl Zeiss.  He was a
friend of Ernst Abbe & told many interesting stories but that was many years
ago & my recall is fuzzy.

">There is real, if minor, increase in the sense of depth that follows this
increase in interobjective distance.

"This factor alone DOUBLES the depth sense - that's more than 'minor'.

"I think the adjective 'cardboarding' is much more descriptive than the
Zeiss term 'plasticity' - an interesting phenomenon that can be observed
when using say standard 7x50 binoculars.  The sense of depth is increased
14 times - 2 by objective separation and 7 by mag.  Objects exhibit only
 2/7 normal 'thickness'

"Many compact binoculars position the porro erecting prisms inward for
compactness.  If I were asked to dream up an advantage other than
compactness I doubt if I would have thought of the theatre use which put
the players closer to the backdrop - I like it!

"Subject of stereo rangefinders for military use.  Fact: Stereo R.F. is
more accurate than coincidence but the operator of the stereo system must
 be carefully selected.  We have experienced how easy it is for some to
freeview a stereo pair, while very difficult for others.  This screening
was missed early on.

"I recall the following story of the WWI naval battle of Jutland in which
the British, using coincidence rangefinders confronted the Germans using
stereo R.F.s.  The first salvos demonstrated the superior accuracy of the
stereo systems, but as the battle progressed the tide changed and the
stereo ranging fell apart.  In subsequent analysis as to why stereo was
best at the beginning but terrible during the heat of battle revealed
the loss of stereo perception by the stereo users.  Under stress they saw
2 images rather than the fused images of a depth scene.  Of course the
coincidence users were not rendered useless by the stress.  I remember
this story from over 50 years ago - my best guess that it was from Dr. Brian
O'Brian who was dean of the Institute of Optics at the University of
Rochester.

"In WWII the accuracy of stereo R.F.'s was doubled w/o increasing their
 base or mag.  They were called 'Ortho-pseudo Range-finders'.  Imagine a
stereo R.F. with a fixed-in-space reticle.  As the convergence angle
between the two images is varied the object ranged upon will move in or
out.  In the complex cluster of prisms which deflect the images to the
eyes one set produces a pseudo image.  The pseudo image can be below
the ortho image so that as the ortho image goes out the pseudo image
comes in at the same rate.  There is no longer any need for a reticle.
Range is accomplished when both images are in the same plane.
Incidentally the pseudo image of the ship is inside out but you can't
expect everything!"

Bill goes on: "Another 'favorite subject' of mine has been multiple
mirror reflections and the resulting image rotation that can ensue.
While teaching 'practical optics' I made a very simple demo with lengths
of aluminum rods connected with short lengths of surgical tubing.  The
flexible tubing allowed the changing of optical axis as if it were a
mirror.  Each length of rod contained a perpendicular orientation
indicator - I used the letter 'F'.  When the train of rods are in a
straight line the letter F's should be aligned w/o rotation.  If you
play with only reflection it will demonstrate image rotation for any
angle.  Two reflections demonstrate no image rotation if the 3rd axis
is parallel with the 1st and independent of the 2nd axis, but if it is
parallel and in the reverse direction of the original axis the image
rotates at double speed.  All angles between can be demonstrated .
There is a problem:  All rotations are counter to those obtained by
mirrors.

"One of my students, Rick Albright (now a prominent telescope maker)
designed and presented me with a mechanical linkage using rods and
universal pulleys rendering correct image directions.  This device
has provided a solution for many situations.

"Another brilliant student, Steve Hines, envisioned a different
practical demonstration which provided more angular information.
Imagine the optical axis receding from you then taking an angular
bend of say 104 deg. back but clockwise 13 deg. then at some other
 set of angles unrelated (maybe) to the original xyz coordinates
and then another mirror.  The problem seems like trying to solve
n simultaneous equations with n unknowns.

"Steve's demo employs the use of 1/4" thick foam core material.
Upon the sheet draw and cut out the complete optical envelope.
This will allow you to represent the mirrors by cutting through
one side of the foam core but not the other as that becomes the
hinge Now you can demonstrate all angles from 0 deg. to 180 deg.
for that mirror-to-axis angle."

I hope I haven't broken the rules by acting as Bill's conduit,
since he isn't officially a member of the list, but I thought
you all might be interested in his comments. Besides, this
list has been rather lean the past few days!

Best regards,
Bill Davis


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