[photo-3d] Re: medium format viewers

["Alan Lewis" <3-d@xxxxxxxx>]

John A. Rupkalvis wrote:
>Not all people have exactly the same eye spacing.  The few millimeters
>variance may seem insignificant, but not to those who are trying to view
>off-axis because of this.

Adjusting your viewer lens spacing when viewing a monolith (single) stereo mounted image does not align your eyes with the image.  It moves the lens centerline to match the infinity centerline of the mounted image.  You are in fact viewing the image off-axis anyway, whether your viewer has adj. lens spacing or fixed.  

This has been discussed at length a few years ago on this list, and I think John Bercovitz made up a nice diagram showing why a viewer does not need adj. lens spacing.

>Perhaps if you are the only one using your viewer, and you only view slides
>that you have mounted yourself, you will find a non-adjustable viewer
>satisfactory.  But, if you ever view other people's slides, or other people
>ever look into your viewer, you should seriously consider getting one with
>an adjustable interocular.

Actually this is an argument for the opposite.  If you are the only one viewing your own slides with your own viewer then you can do anything you want regarding mounting practices and viewer design.  You only have to satisfy yourself.
If you view other peoples slides or they view yours, what is needed is to mount all slides to a standard, usually by using a mounting gauge.  Having adj. lens spacing is only an attempt to fix a mounting problem.

There is one condition where adj. lens spacing is useful.  This is when you have twin slides (separate) and a viewer that can move both the slide and lens with each other (two single viewers linked together).  You are then truly adjusting the lens spacing to match your eye spacing.
This is not the case when viewing monolith stereo mounted slides though.


>Several years ago, I had a table at a local camera show.  The fellow with a
>table across from me had a non-adjustable stereo viewer, that he was
>demonstrating with some stereo slides that he had brought with him.   It
>surprised me to see how many people looked into the viewer, and then closed
>one eye!   When he told them to look with both eyes, common responses were
>"I can't, it hurts my eyes", or "but then I see two pictures".  I often
>wonder if such viewers were at least partially responsible for the decline
>in stereo photography after the 1950's.

I would say that it is more likely that the need for adj. lens spacing in the viewers to fix mounting variances was one problem with the popularity, but not the only one.

All this being said, there is no problem with using or making a viewer that has adj. lens spacing.  It is a feature that does no harm to the apparatus, and it may offer a temporary fix for non-standard mounting practices.  But it is absolutely not necessary in the case of viewing monolith stereo slides if the viewer design follows the same standard as the mounting gauge for infinity spacing.

I mount all my slides (35mm, Realist, MF) using commercially available mounting gauges.  I design all my viewers around the same mounting gauges.  I never have a problem with comfortably viewing my slides and anyone else's slides mounted to a standard.  And no one has problems with mine.  Everyone can freely share stereo slides knowing that no one will have difficulty viewing them.

The debate is very similar to the one we have had on this list about adj. slide projectors during a slide show in order to fix mounting problems (non-standard mounting practices).  It is not desirable to be fiddling with slide projectors for every slide, just as it not desirable to fiddle with the viewer lens spacing for every slide.

Who knows, maybe someday I'll make a viewer with adj. lens spacing.  If people really think they need the feature then I may offer it.  The customer is always right.



Alan Lewis
3-d@xxxxxxxx
Serious viewers for serious Viewers
http://members.home.net/3-d


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