T3D Tabletops by the numbers

[john bercovitz <bercov@xxxxxxxxxxx>]

If you download an OCC entry form from 
http://www.angelfire.com/ca2/tech3d/pix.html
and read it, you'll see:

> SPECIAL SLIDE CATEGORIES
> 
> Contemporary --- Category 'C' ---- PSA Stereo Division Silver Medal
> Tabletop ------- Category 'T' ---- Oakland Camera Club Medal

and:

> Selectors sit nine feet (2.7 m) from the screen and the image height 
> of a 24 mm-high slide aperture on the screen is 56 inches (1.4 m)

>From the last statement, we can quickly calculate the focal length of 
the lens we need to do the tabletop if we don't intentionally want to
put stretch or squash in our stereo pair: 9'/56" = 1.9  If the image is 
24 mm high, then the focal length would be 1.9*24 = 46 mm so you Minolta 
owners have it made.  8-)  Actually, this is the distance of the lens 
from the film rather than the actual focal length but they are nearly 
the same except for really close closeups.  

[ A really close closeup might be 150 mm (6") away in which case a 
35 mm focal length lens would sit 1/[1/35 - 1/150] = 46 mm away.  No, 
I never dry lab these things.  Yeah, right.  8-) ]

Next download Tom's spreadsheet, MAOFD for dummies, from
 http://webman.mc.yu.edu/t3d  

Now we have to decide if we want a reproduction scale of 1:1 or if we 
want to exaggerate or minimize the scene.  Let's take the example Tom 
already has in the spreadsheet.  Focal length is 50, near point is 30" 
and far point is 40".  As we can see, we can shift the camera up to 
3.8" without exceeding the comfort limit for most observers.  If we 
actually shoot at 3.8 inches, we will maximize depth cues but we will 
also make the pair reproduce at 2.5/3.8 = 70% scale.  So it will look 
smaller than real life.  You may want to do this and you may not.  If 
you want to reproduce at normal scale, then set shift to around 2.5".  
If you want to make the thing look larger than real, set to something 
less than 2.5".  Make the shift 1.25" to double the reproduced size.

OK, so let's say we want to shoot at full scale.  Then we are allowed 
more depth than from 30" to 40" if you want it.  You can fool with 
either number until the shift goes down to 2.5"; your choice.  I will 
increase the 40" number.  When I increase it to 48", the camera shift 
drops to 2.5".  So I can have a tabletop extending from 30" all the way 
to 48" if I want and still not exceed MAOFD.  

Notice that Tom's spreadsheet also very kindly gives you the best 
focussing distance and it is 36.9 inches in this particular case.  
So on your table top, set up a monument (a chess pawn, for instance) at 
a distance of 36.9" from the apparent position of the iris of the lens, 
focus on it, and Bob's your uncle.

John B


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