P3D Re: stereo mount

[aifxtony@xxxxxxx (Tony Alderson)]

nathan kreuter wrote (digest 3405):
>I'm trying to make a mount for a pair of Canon QL-17 rangefinders.
>Mounting them is straightforward enough, what i'm having trouble with is
>acoounting for error on the right and left sides of each stereo frame.
>With my existing mount the image starts and stops at different points b/c
>both cameras are pointing directly forward. I am aware that by "towing in"
>the lenses towards each other (1-5 degreees depending upon distance of
>subject) the error can be negated.<

Oh, don't call this an "error"! You'll offend us parallel fetishists ;-)

Because the "error" --keystone distortion-- occurs when one compensates for
the incomplete image overlap by toeing-in the cameras.

> (...): Anyone got a better idea<

Well, you could remount your chips in narrower masks and reset the window ;-)

Ah, but you want to maintain the full frame. OK, but in my opinion you
should keep your toe-in under 3 degrees (not 5).  Anyway, your mechanical
idea seems like overkill to me. Why not just attach one camera to the bar
with a wing nut, and loosen it to nudge the toe-in and retighten. There is
no compelling reason to angle in both cameras, you can mount one square and
adjust only the other. You could scribe a line(s) on the bar to show your
range of toe-in, from 0 to 3 degrees (you might want to run the math and
mark distances to your convergence point).  As a practical matter, you are
unlikely to be readjusting convergence for every shot.

I've seen very elaborate rigs for this purpose, with fine-screw adjusting
and protractors and so on, but mostly they just cost more money.  I suggest
you build a simple rig, take a bunch of pictures, then evaluate your
experience and decide if you need something more. Practical experience will
give you better ideas for your next setup.

Tony Alderson



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