Re: Hyperstereo threshold

[Joel Alpers <joel.alpers@xxxxxxxxxxx>]

john bercovitz wrote:
> 
> > Only one warning! I am new at this, and I probably breaking all
> > kind of rules but there is no one around to tell me that I am wrong.
> 
> No rules are being broken here.  In fact, your observations go right
> down the line with the rules.  The nominal rule is 7' for standard
> separation but that's only nominal.  In most situations you can get
> much closer.  The 7' would scale up linearly with the increased
> stereobase if I remember correctly.
> 
> John B


Ha ha! Now here's a REAL switch - John B answered a question in English,
and I provide the formula!!!!


Here's the formula for stereo base:


          D * S1 * S2
    B = ---------------
         F * (S2 - S1)

Where:

    B  = Base separation
    D  = Deviation on film
    F  = focal length of lens
    S1 = distance to near object in picture
    S2 = distance to far object in picture.

For medium format, D is about 2.5mm. Re-arranging to get the near
point, I get (check me!)

   
          B * F * S2
   S1 = ------------------
        (D * S2) + (B * F)

Usually you include infinity in the photo, so as S2 -> infinity,
the (B*F) term becomes insignificant, the S2's cancel out, and
we're left with the simpler:

         B * F
  S1 = --------- 
           D

For the 80mm 124G lenses 90mm apart, I get  90*80/2.5 = 2.88 meters,
or almost 9 1/2 feet.

At 8 feet (with infinity in the photo)  on-film deviation goes up to
just over 2.95mm, and at 7 feet, it goes to 3.37.  You can probably
fudge with this -some-, but the larger you make it, the bigger a
chunk you're cutting out of the people who will be able to view
the slide without eyestrain. I break this "rule" at times, but when
I do I'm careful who I show the slides to.

Do some experimenting with your single camera! Perhaps a piece
of angle iron with two marks 90mm apart -- take a photo of a 
scene (including infinity) and put a near object 9', 8', 7', 6'
away. It would make a fun entry for the MF folio...

Interesting that you bring this up right now, as yesterday for
fun I was working on a CGI WEB program to allow someone to plug
differnt numbers into the formula(s) above. Thought it might make
a neat learning tool for the "tutorial" section of the RMM WEB page...

Joel.