Re: Composing for window effects

[P3D Dr. George A. Themelis <DrT-3d@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

We continue with more interesting questions, courtesy of Paul Talbot:

>>I only use 4P mounts if I want to crop the picture, not for setting
>>the stereo window.  To achieve this wide range the Realist must not 
>>have significant frame overlapping which steals away image space.
>    
>Aha!  I suppose we can expand this comment to say the images must not
>have been cut in a sloppy fashion that caused loss of image space, as
>well!  In ripping apart a number of Kodak-mounted slides, this was a
>frequent problem.  The full range of chip spacing adjustment often was
>not available.

Yes, that's true.  I have seen this situation with Kodak-mounted slides.
Somehow they do not do a good job cutting the chips right at the center
between two sprocket holes.  It is then very difficult the adjust the
window with the 5p mount because there is no room left!

>What causes variation in frame overlapping other than the focus setting?

I don't think the focus has anything to do with it.  Frame overlapping
is a common problem in Realists but also other stereo cameras as well.
I don't know what causes it in the Realist (if I knew I could be fixing
it!)  Slack in the sprocket gears?  Any ideas anyone?

>> For example, can you use a 5p mask to mount a picture with 2 to 2.6
>> feet range of depth?  The answer is no.  

>I've heard mention of problems with very close Realist pictures.  Can
>you elaborate?

This is also similar to taking a close-up with an SLR and parallel
shift (no convergence) or also in aerial photos.  Depending on the
magnification, FL and amount of shift you only get partial overlap.
The areas at the edges that does not overlap (sometimes called "ghost
bands") need to be masked out to set the proper window (if not masked
out then the window is "beyond infinity")  The room that you have
to work with is equal to the image size minus the mount size (if your
camera shows overlapping or if the chips are not cut properly then
you have reduced your image size).  In macro SLR shots I usually
have enough room to mount a full frame pair in 7p mounts.  But when
using the Realist, there is precious little space to mount close-up
images in 5p mounts.  The 4 ft. number that I mentioned above game
out of my head from experience but it can be calculated if you know
the film and mask sizes.

-- George Themelis


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