Re: Mounting

[P3D Robert Thorpe <thorpe@xxxxxxxx> To: 3D List <photo-3d@xxxxxxxxxxxxxxx>]

I would like to make a few comments on mounting prints on Holmes cards. Some of you have visited my page on mounting the cards, those that haven't can find it at:

http://www.netins.net/showcase/visres/3D/holmscrd.htm 

As George mentioned, there are two schools of thought; mounting to infinity and mounting to the window.

I think that if you have to choose, mounting to the window is much, much more important. But with prints you can easily have it both ways if you want to. This does not apply to Q-Vue cards however. 

INFINITY SPACING

The problem with mounting to infinity is that every viewer is a little bit different. This is not much of a problem with slides because most viewers are set up with Realist spacing on-center (I assume this is the case, I can't swear to it). But with Holmes-style stereoscopes, every one I have ever tried has a different on-center spacing than the others. 3 1/2 inches is sort of the standard, but the ACTUAL infinity spacing is determined by:

- the original diameter of the eyepiece lenses - the method and width that they were cut to when they were cut in 
half and squared up
- whether they were mounted in the steroscope perpendicular to the 
central axis or canted slightly wall-eyed or cross-eyed - and the final center-to-center spacing of the viewer lenses. 

You can make a quick check of these things by pointing the axis of your stereoscope directly at the sun with the lenses on the side towards the sun. Put in a blank card and move it back and forth till the sun projects a pinpoint of light (the old smoking ant trick). I'm not saying this is your infinity spacing but you will notice that the distance between the pinpoints varies from viewer to viewer. 

Why is this like this at all. My THEORY is that the printmakers of the last century wanted to make the cards with photos as large as possible for more detailed viewing. Since there was no ASCII, ANSI, ASA, ISO or other standard, they made cards to match their own viewers (maybe). Every company had a different standard. If you even want to worry about infinity spacing, you are going to need a matching viewer for every brand of stereocard you own. My advice is if you want to mount the cards yourself and you want infinity spacing either go to the 3 1/2 inch "standard" or match your favorite or most long-lasting viewer. You will probably never notice a difference anyway. More on this in a minute. 

STEREO WINDOW SPACING

My trick for placing the stereo window is to put one of the prints on top of the other one and slide it up towards the top of them both, just a little. Find the object that is closest to you at the bottom of the print and line the two prints up so the closest object in the top print is right on top of the one on the bottom print. Slide the top print back down so the bottoms of the prints are lined up. The two prints should be offset a little bit left and right. I lay a template the size of the final mounted print on top of the two views and cut around it with an exacto knife. This gives a stereo window that is even, front to back, with the closest object. There is a little more to this of course, but that's the idea.

HAVING YOUR CAKE AND EATING IT TOO

These two approaches are not mutually exclusive. Trim your prints to get the correct stereo window spacing, then move them closer or farther apart till the infinity objects are the standard distance apart (3 1/2"?). The placement of the subject in the trimmed print determines the stereo window, the placement of the prints on the card determines the infinity spacing. The septum (the space between the views) must be variable, which may be more or less offensive to your personal tastes. This is why fixed distance slide mounts and Q-Vue cards won't work with this double approach. With them, you must use one approach or the other. 

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Before I figured this out, I trimmed my cards out by the Stereo Window method and just mounted them about an 1/8 inch apart. My prints are only about 2 5/8 inches wide, so you can see I was way far off from the infinity spacing. I haven't noticed any problem with this whatsoever. Your eyes naturally converge and diverge and accomdate this change. If infinity spacing was so important, when you free viewed a stereocard with 3 1/2 inch frames, your mind would really object. it would demand prints no wider than the separation of your eyes. 

Robert Thorpe
Cedar Rapids


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