[MF3D.FORUM:637] Re: Image overlap

[Greg Erker <erker@xxxxxxxxxxxxxx>]

>The amount of the "unrelated" scene
> is equal to the deviation between near point *and infinity*
>or, in my terminology FB/I (F: Focal length, B: Stereo Base, I:
>distance of near point).  As you come closer (I increases) then
>the unrelated scene increases.  Large focal lengths and stereo
>bases work against you too.

  Interesting. Let's see how this works. 80mm lens,
60mm stereo base and 2 metres to near point. So 60*80/2000
= 2.4mm.  Seems to match the (ball park) on film deviation.

  For closeup and big stereo base: 80mm lens, 100mm base,
1 metre near. Non-stereo film amount is 8000/1000 or 8mm.



>Another way to write this (useful in ultra close-ups) is M*B
>where M is the magnification.  If you are working at M=1
>and you shift the camera by, say, 10mm, then you just
>"lost" 10mm of image overlap.  If you shift 36mm with a
>35mm camera at M=1 then the right and left views will be
>totally unrelated (not a very appealing stereo pair).

  Cool too. We discussed this method problem
on P3D one time.

>As you come closer and closer, image overlap (or lack of
>it) becomes a real problem and you are forced to use
>tilt to record the stereo pair.  The good news is that
>keystone distortion is reduced because
>the effective FL becomes M*F (M: magnification) so it is
>really large.  SEM stereo pairs with tilt at 1,000 or more
>show no keystone distortion from a practical point of view.

  Is this correct?  At mag of 1 the 80mm lens
is extended 80mm from the infinity focus point.
So it acts like 160mm. So shouldn't your formula
be (M+1)*F?

  Great stuff George. Maybe some Tech-3ders
would like to see this.

Greg E.