Re: Franka Beam Splitter

[John Roberts <roberts@xxxxxxxxxxxxxxxxx>]

>Date: Tue, 21 Nov 1995 14:59:42 -0600
>From: michaelk@xxxxxxxxxxx (Michael Kersenbrock)
>Subject: Re: Franka Beam Splitter

>>...if your camera has a horizontal-travel
>>focal plane shutter, then when you use a beamsplitter the
>>synchronization
>>between the left and right views will be only a little better than 1/2
>>of the X-Sync speed.

>I assume that "half the speed" means the "twice as fast as" sort of
>interpretation seeing as how a horizontal curtain has half-a-full-35mm-image 
>distance to travel between the "same points" in each image.

That's what I meant. "Shutter speed" is specified as a time interval. From
previous posts, I understand that the shutter travel time can be expected
to be somewhat less than the X-Sync period, but more than half of that period -
for instance, a camera with an advertised X-Sync of 1/60 second may have a
shutter travel time of 1/80 to 1/100 second. The homologous points in a
beamsplitter pair would be "about" half a frame width apart, so in the case
above the synchronization between the two images would be about 1/160 to
1/200 second (which is marginal for some subjects).

>I wonder if having out-of-sync but sharp is better or worse than in-sync-
>but-fuzzy?   I expect that the direction of movement vs. direction of the
>curtain would make a difference in quality as well.  

It can make a considerable difference. In most cases, you get the best results
if the direction of motion of the subject is perpendicular to the direction
of motion of the (focal plane) shutter. If you have a horizontally-moving
subject and a horizontal-travel shutter, then the subject can appear elongated
or shortened, depending on the relative directions of motion. If you are also
using a beamsplitter, you could also get a distortion in the apparent depth
(for the same reason that the Pulfrich effect works).

John R


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