Re: [photo-3d] CU Mounts?

["Dr. George A. Themelis" <drt-3d@xxxxxxx>]

At 12:26 AM 2/21/01 EST, John Ladd wrote:

>if I'm using my 35mm SLR on a slide bar to shoot macros 
>can I use 7 perf mounts, or do I need to go with 
>a narrower opening?

It depends on the magnification and the stereo base.

(It also depends on whether you converge your camera
or not... I will assume that you do not converge the
camera but just slide it parallel, so the stereo base
is the amount of shift)

The only purpose served by these narrow mounts is
to cutoff parts of the image so you can position
the correct stereo window.  When you shift the
camera parallel, the stereo window is automatically
positioned at infinity.  So you must correct it,
no matter what your subject is.  This is done
by blocking the left side of the left image and
the right side of the right image.  In close ups
there is more cropping that needs to be done, as
compared to normal stereos.

The equation that gives the amount of image that
needs to be cropped in close ups is: M x B, where 
M is the magnification and B is the stereo base.

So, for example, if you are shooting at 1x magnification
and shift the camera by 6mm, you will need to crop
6mm.  The full 35mm frame is 36mm wide.  The width of 
the 7 perf mount is 28mm.  That gives you 8mm of 
cropping so you are OK using the 7 perf mount.

In general, using a 7 perf mount to crop full frame
stereo pairs gives you plenty of space.  It is only
in extreme situations that you will need something
more narrow.  If that happens, you can try a 5 perf
(realist) mount.  

Note that the Spicer mounts offer a close-up 7 
perf mount in case you feel that going from 7p to 5p 
is a big jump.  The primary use of this mounts is
for close ups taken with 7p cameras.  The Spicer
mounts also offer a close-up 5p mount.  The next
step is the 4p mount.  If you are using aluminum
mounts, you can overlap two of them for variable
size cropping.

George Themelis

 

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