Viewer vs. projection - alignment & depth range

[P3D Dr. George A. Themelis <fj834@xxxxxxxxxxxxxxxxxxxxx>]

Dave Worrell, in his discussion of viewer vs. projection, mixes two
different things: mounting and depth range.  These should be kept separate
because bad mounting (vertical misalignment specifically) can be tolerated
in projection by adjusting the vertical separation of the images while it
cannot be tolerated in the viewer.  One the other hand, excessive depth
(Dave's main problem) can be better tolerated in the viewer than in
projection.

Dave says that he has not be paying much attention to mounting and was
wondering what all this fuss is about.  Perhaps his eyes have more
tolerance for vertical misalignment than mine because it is in the viewer
that I see the problem and, if excessive, I cannot tolerate it.  One thing
that bothers me is if the clean edges of the aluminum mount do not
intersect the same objects in both views (sign of vertical misalignment,
_assuming_ that the mount is OK) and I use the edges to make sure that the
slides are aligned.  In projection, if you look over the polarizing glasses
you can see the vertical misalignment and correct it.  But that's not a
good practice.

Depth range is a different animal.  If the separation of the infinity
points (object that is the furthest away) in the screen are past the
spacing of the eyes then the eyes will have to diverge to view infinity. 
Some people have or have developed tolerance to this (to a certain degree)
but for most people it is a problem.   That appears to be part of Dave's
problem.  The magnitude of the problem depends on factors like size of
projected image, distance of the viewer from screen and mounting technique.

If a slide is going to be projected, the advice usually given is to use
less depth range in the scene.  With the Realist and infinity present, do
not put anything closer than 7 ft.  In  hyperstereos adhere to the 1/30 rule,
or (more conservative) 1/50 rule.  Charles Piper has made a connection
between the depth of field and depth range (governed by the same inverse
distance law).  He says that you should limit your scene between the 5.6 (I
think) marks in your DOF scale.  More depth can be tolerated in the viewer
so if you shoot for a viewer you don't have to follow these conservative
recommendations.  These recommendations however become important in
projection and, especially, in large screen projection.

>What do I look for in a slide for projection? 

Limit your depth range as discussed above.

>When mounting, what should I look for in
>the viewer as a clue to how it will perform when projected?

Make sure that it is aligned properly.  The slip-in mounts that you are
using offer almost no control over horizontal spacing and vertical
misalignments are common.  If you cannot tell alignment in the viewer, use
a gauge.  I am very good in detecting alignment problem in the viewer and
have not seen the need for a gauge.  I have managed to successfully
sandwich two stereo slides, one of which was taken with twin SLRs, and
align them with trial and error.  That's a lot of trial and error!

George Themelis


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End of PHOTO-3D Digest 1748
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