Re: [photo-3d] Re: Misalignments & stereo projection

[Project3D@xxxxxxx]

In a message dated 01/04/01 00:43:14 GMT Daylight Time, John Goodman wrote:

<< Bob Aldridge wrote:
 
 > In a perfect world there would be no vertical misalignment
 > in the slide mounting. 
 
 Am I correct in assuming that the "vertical misalignment" being 
 discussed means misalignment between the left and right 
 mount apertures, and *not* between the homologous points in 
 the image? I fully appreciate the importance of having the 
 mount aperture windows appear congruent on the screen, but 
 assume it is bad practice to adjust for vertical *mounting* 
 errors (misaligned homologous points between the two chips), 
 because this almost always ruins the on screen congruence of 
 the mount apertures.>>

For comfortable viewing of the image, the homologues should be at the same 
height on the screen.

Unfortunately, three things can make one homologue higher than the other.

1.  Bad mounting
2.  Bad mounts with one aperture higher than the other. 
3.  Bad projection

The last one is easy to fix. Hire another projectionist :-) :-) :-)

2. will rarely be a problem - except with shows made up of slides from a 
number of authors.  The RBT mounts have gone a long way to eliminate mount 
variations - particularly if you take care to put the two halves together 
with the little groove on the top of both halves. Spicer mounts are also very 
accurately made, and therefore consistent within each batch. The problem 
comes with slides with a foil sndwich.

Here, unless the foil is EXACTLY the same height as the glass or card covers, 
it will be able to go out of alignment. So, as the slide sits in the 
projector on the edge of the COVERS, if the foil is crooked inside - even 
1/10 mm - it will be out of alignment on the screen.

In all these cases, adjusting the projector will correct it, and the 
apertures will be congruent on the screen.

With bad mounting, the height of the homologues will differ relative to the 
apertures. If the projector is adjusted, the homologues will become level on 
the screen, but the frames will not be congruent. The audience will see a 
double edge at top and bottom. Whilst they may not actually be aware of the 
reasons, the "soft" edge will not look "clean" and it will detract from the 
overall impressioin.

So, as to the question of adjustments during projection, it would be good if 
there were none, and in one's own personal shows this is easily achievable 
with care in the mounting. With mixed shows, being realistic, there will 
never be a time when slides are projected with a constant alignment unless 
freedom of choice of mounts is removed from authors. Under these 
circumstances, the projectionist should adjust the vertical alignment _of the 
homologues_ smoothly and decisively as soon as the slid is on the screen, and 
then focus it. This reduces the discomfort experienced by the audience to a 
minimum - they may not even be aware of the misalignment because they only 
get to see it on out of focus slides...

Bob Aldrtidge
Stereoscopic Society Projectionist 

 

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