T3D add'l maofd spreadsheets

[john bercovitz <bercov@xxxxxxxxxxx>]

I was corresponding with Jon Golden; he's experimenting with closeups 
in medium format with a twinned setup.  That got me to thinking about  
the appropriateness of the existing mafod spreadsheet for his situation.

I've just devised two new maofd spreadsheets.  The old maofd 
spreadsheet is most suitable for those who use slide bars because if 
you input the nearest and farthest distances in your scene, it will 
tell you what the maximum stereobase for your slidebar should be.  

The new maofd spreadsheets take different inputs and give different 
outputs.  The spreadsheet called "anafa.xls" gives you the near dis-
tance and the far distance if you give it the distance at which you've 
focussed your lens and the stereobase of your camera.  The spreadsheet 
called "afaan.xls" gives you the far distance and the distance to set 
your lens focus at if you give it the near distance and the stereobase 
of your camera.  I did not do a spreadsheet for an input of a far 
distance.  Of course if your far distance is infinity, then the 
conventional 1 in 30 rule applies and the near distance is 30 times 
your stereobase.

These new spreadsheets should be useful to those with stereo cameras 
and to those with twin rigs.  They should also be useful to those who 
use a single camera on a slidebar but who like to stay as close to the 
nominal stereobase of 65 mm as they can.

These spreadsheets are temporarily located in the T3D web site Joel 
Alpers of Rocky Mountain Memories maintains for us at:
http://www.frii.com/~rkymtmem/tech3d/tech3d.html

These spreadsheets are expected to soon be located permanently at:
http://calcite.rocky.edu/ftpdir/photo/photo-3d/technical/maofd/

The variables in the spreadsheets are:
f = focal length of your stereo system
d = maximum allowable on film deviation (maofd)
    Usually this will be f/30 but it can be as much as f/15
    if you have a scene which gently moves from near to far 
    and has no near objects partially occluding distant objects.*
b0 = the camera's stereobase
a = best focus distance to get af and an equally into focus
an = distance to nearest point in scene.
af = distance to farthest point in scene

Here is a sample output from anafa.xls:

  f        d        b0      a      an      af			
input	   input  	  input	 input		output	 output			
 50      1.67      65.0    250     238     264			
 50      1.67      65.0    300     282     321			
 50      1.67      65.0    350     325     379			
 50      1.67      65.0    400     367     439			
 50      1.67      65.0    450     408     501			
 50      1.67      65.0    500     448     565			
 50      1.67      65.0    550     488     631			

* This 1/30 to 1/15 is the ideal and is what you should stick to in
order to make slides of all types viewable by people of all abilities.
_However_!  Don't forget that you will play havoc with projection if
you violate an existing standard.  For instance, the Realist standard 
specifies something like 1.2 mm deviation from the window to infinity.
If you go ahead and use 1.67 mm deviation because you used 50 mm 
lenses in the camera, then the projectionist will have to readjust
the projector for your slides or else the infinity homologues may
be too far apart on screen for all of the audience to be able to fuse
them.  In this case, it is probably better to stick with the 1.2 mm
maofd even though 1.67 mm would be perfectly viewable.  Of course you
can use some of this difference between 1.67 and 1.2 to poke objects
through the window if that amuses.  You should not encounter this 
problem in hand held viewers or on screen if you are able to diverge
comfortably.  You also should not encounter this problem in hand held 
viewers if your IPD is in the upper half of the range.

John B


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End of TECH-3D Digest 270
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