[MF3D.FORUM:347] Re: BASE = 1 in (FL of Lens) "rule"

["Oleg Vorobyoff" <olegv@xxxxxxxxxxxxx>]

Hello Stuart,

What you say makes a great deal of sense to me. But I do have a clarification.
The rule I use for getting stereo base for MF is NP/30 x 70/FL.  Both the rule
and the exact formula (see below) tell you that anything closer to your Hassy
rig than about 10 feet will poke through the window if 1) your shot contains
distant objects and 2) the stereo pair is conventionally mounted. By
conventional mounting I mean that the most distant object is set at 24mm
deviation relative to the frame.  This does not mean that a scene with objects
in it closer than 10 feet will not work.  As you point out, it will often work
very well.  But the rule does tell you that there is a chance you will end up
with bad window violations uncorrectible during mounting.  That is an important
"heads-up" for the photographer.  Before I started using the rule I had
accumulated a box full of barely viewable and absolutely unprojectible slides,
not to speak of many boxes of overly flat slides.

Calculation details for an MF shot with 80mm lens, 10.8’(3.3m) near point and,
say, a 100m far point are as follows:
 Base = 0.0024*(100*3.3/(100-3.3))*(1/0.08-1/5) = 0.101m
By the MF 1/30 rule:
 Base = 3.3/30*70/80 = 0.96m

Oleg Vorobyoff

-----Original Message-----
From: Stuart Johnson <gcsphoto@xxxxxxxxxxxxx>
To: Medium Format 3D Photography <MF3D.Forum@xxxxxxxxxxxxx>
Date: Tuesday, March 28, 2000 5:55 PM
Subject: [MF3D.FORUM:333] Re: BASE = 1 in (FL of Lens) "rule"


>Hello Bill and the rest of MF... This is in response to Bill's requests and
>my reviews
>of recent MF emails....  If I'm "way off" with my opinions, please let me
>know....
>
>I started to write to you with my opinion and after I read Tom's and others
>"formula's" I realized that in my opinion.... That these formula's may not
>be right!   The MAIN thing is to capture an image that looks NORMAL.  Our
>eyes do NOT shift left/right in directions -only the eyeballs have an
>ability to turn "in".  They do this all the time.  When you place the lenses
>FURTHER apart, you get EXAGGERATED stereo to some degree (from minimal to
>extreme).  I can't agree with this formula that some seem to be fixed upon
>at 1 in 30 or 1 in (FL of Lens). Why?  Because my experience has shown that
>I can have a perfectly NORMAL 3D photo where my foreground is only 3 feet
>away!  Also, that I can compensate for the Stereo Window placement by how I
>trim and mount my Prints. Finally, if Tom's formula is correct ( Base = 1 in
>FL of Lens ) then if I have a 100mm Base with my Hassy's and use 80mm
>lenses, then the CLOSEST I can have my foreground is...  26 feet.  This
>can't be right....  If it is, how would I ever be able to take a portrait in
>stereo?
>
>For example:  You can set up two 35mm cameras with 50mm lenses at 70mm base
>and have foreground as close as 4 feet and take a beautiful stereograph.
>According to some, this is not supposed to work! You can also tilt the
>lenses IN a little (5 degrees) and limit the background (no infinity) and do
>the same thing. This too is not supposed to work according to some.
>
>Now, when you exaggerate the BASE then the images tend to look...
>exaggerated.  Mainly the objects get smaller, and tend to 'lift off' the
>picture...
>
>You may want to check out the tables in WAACK's book (from Reel 3D) or
>Ferwerda's book (Expensive if you can find it).   What are our eyes focal
>length? It must be quite small....
>
>Finally,  I shoot in Prints and it appears that the viewing of prints is
>different then viewing the Medium Format SLIDES.  Perhaps someone with
>experience with bot formats can better answer your question...
>
>Good Luck!    -I've got a lot more reading to do!
>
> -Stuart
>