P3D Stretcho-TDC

[Michael Georgoff <Michael_Georgoff@xxxxxxxxxxxxxx>]

I'm suffering from severe z-axis stretch and I just do not know what to do
about it.  

Once upon a time, I  went to a meeting of the Oakland Camera Club, Stereo
Division.  There I saw a slide show of Realist format stereo, and in
retrospect, I'm thinking it was probably a TDC 716 projector.  It was my
first stereo slide show, and I was astounded.  Before the show was over, I
was wiping tears of joy and amazement off the insides of my polarized
glasses.  Later in life, I saw my second slide stereo show, this one by Mark
Blum, doing this "Under the Sea" show with twin 2x2 projectors, and I think
they were probably the not-cheap Kodak EktaBucks something or other
projectors.  Once again, I was astounded, amazed and thrilled to stereo
death with it all.

Lately (last week to be exact) a TDC 116 showed up on my doorstep, thanks to
the miracle of savings accounts and watching S3D like a hawk.  Finally, true
stereo projection in my own home!  Joy!  Rapture!  But wait! Z-axis stretch
from hell!

The TDC 116 has 5" lenses on it.  These length lenses are not optimal for my
use, which is really home viewing and projection for a very small group
(annoying family and friends, pets, etc, with my slides, which I personally
enjoy the hell out of, thank you very much...).   I understand that there is
a direct relationship between the focal length of the lenses, the throw
(distance from projector to screen) and the image size on the screen.
Someone kindly shared with me this formula: (film aperture width times
throw)/lens focal length = image on-screen width.  And, looking at the info
that's been published about the wonderful Brackett projectors, there are
charts that show what kind of projected image height you can expect given a
lens of x focal length at y distance from the screen.

Okay, so what's problem?  Well, me being the projectionist, I've gotta be
the farthest human from the screen, so the image which is already small by
virtue of having 5-inch lenses is even smaller given the distance away that
I am, lonely projector guy way in the back...  Okay, so why not just quit
whining and get, say, 4-inch lenses?  Or, better yet, 3-inch lenses!  Well,
there were/are stock standard TDC manufactured 4-inch lenses given your
willingness to part with more hard earned cash.  There are not 3-inch lenses
and per my fooling around with the TDC it looks as though it will only
accept 3-inch lenses if you do some serious mods to the lens housing shell
whatever-you-call-it casting, like cut the back half of the damn thing off.
(Or, I guess you could mount them inside the housing instead of
outside...has anyone ever tried this?  Downside could be vingnetting from
the housing openings?)  Okay, so far this is just complaining about how the
image is smaller than I'd like to see. Or is this related?

The real problem is that the stretch is horrible.  (And I thought I had
distortion shooting with the Realist's 35.2 mm lenses and viewing with the
Kodaslide's 47mm focal length!).  I can hear some out there saying "So the
stretch is bad from the projector position...  that's not the optimal
viewing position, you ninny!"   Okey-dokey, being a curious sort, I slowly
walk up closer to the screen, and it STILL does not look approximately right
(no z distortion) until I'm 3 or 4 feet away.  Huh?  It appears that ortho
is 3 or 4 foot away.  Izzat right?  5" lenses, 15 foot throw, ortho = 3 or 4
feet?

Therefore:

1. What, if any, is the relationship between the focal length of the
projector lens and the amount of z-axis distortion?

2. How is it determined what's the best "ortho" seat in the house?  In other
words, how do you calculate best viewing distance?  I have seen
approximations of best seat = 2x or 3x the on-screen image height

3. Is there any relationship between question #1 and #2 above?

What I really want out of all this is natural viewing, I want out of z-axis
stretch hell...  (I'd have searched the P3D archives to look for clues, but
they are offline for spell, as Bob's got live alligators to wrestle.)  Any
and all education welcome, on list or off.   I will even accept long-winded
math if that helps me understand..

Thanks!

Michael Georgoff
San Jose, CA

(I don't have my "ortho uber alles" armband on today, but I want some
reasonable reality here..or I might just give up and take Phil Spector's
advice and go "back to mono"...)


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End of PHOTO-3D Digest 3235
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