Mounting slides or prints for a stereo viewer or projector

by John Wattie



Mounting stereoscopic slides for projection introduces a further window problem which does not worry computer viewing because of the strange ocular gymnastics used to see the images.

The most distant objects in the stereo pair must not be separated by more than the inter-ocular distance.

When mounting the stereo slides, a jig of some kind is very helpful. 

  1. Set up the most distant points at 63mm separation. 
    a)  The slide mounts are placed the same distance apart they will be separated in the viewer. 
    c)  Use a home-made jig over an illuminated view-box to keep the mounts precisely aligned.
    d)  Wear 2 diopter or stronger hobby glasses - they improve precision and also work as stereo viewers.
    e)  Homologous furthest points are located on each of the pairs - use stereo viewing to locate them.
    f)  The distance between these far points is measured precisely with dial callipers and set at 63mm.

  2. Ensure the slides are not tilted - the pictures must be aligned exactly along the line they were in when taken, parallel to the bottom of  the slide. Otherwise your eyes will have to move vertically or rotate in their sockets to fuse the images, which is possible, but many people in the audience will not be able to do it.

  3. Choose a pair of windows separated by the distance between the nearest object  on the two slides or very slightly less. This means you will need a choice of slide mount windows.

  4. If the window edges are further apart than the nearest object, then that object will seem to project through the window. This "poke in the eye" technique is fine, but should not occur from the edges of the picture, as discussed above. 

  5. Special window shapes and arrangements where one edge is closer than the other can be set up, including the "falling frame" to reduce edge stereo failure. This means you will have to cut your own stereo windows, which is not easy. (Some purists object to non-standard window shapes. Usually they do not realise that edge stereo failure is more annoying than a slightly unusual window shape, which many viewers will not notice anyway).

  6. To bring the window closer, as is often desirable for close up stereo, mask down each outer side of the stereo pair. This can be done with either separate 35mm slides or dedicated stereo camera pairs. It is possible to include infinity objects, but they must still be placed 63mm apart.

  7. Commercial masks adjusted for window are available. These masks are no good for images bigger than the Stereo-Realist reduced size format. They spoil full size 35mm frames produced by shifting a single camera, as advocated here. These masks are not designed to include infinity with nearby objects.


35mm or 6x6cm transparencies mounted together in a single slide mount can only be projected in a dedicated stereo projector. 

Only some stereo projectors will handle full frame 35mm, and the author has never seen a  stereo projector for 6x6cm format. 

It is easier to project these big, separately mounted slides using two standard projectors. Each projector will need a polarising filter properly aligned to match the polarising glasses worn by the audience. The screen must be metallic, to avoid scrambling the polarised light. A board painted with aluminium paint will do the trick in an emergency. A parabolic aluminium surface is ideal (e.g. the projection screen used in some front projection TV systems, or a Kodak Ektar screen). The parabolic screen gives a very bright image but with a narrow viewing angle. That is ideal  since the acceptable viewing zone for stereo is in a 60 degree - just the same as the brightest image from the screen.

In a viewer, ortho-stereo occurs when the focal length of the taking lens is the same as the viewer. In stereoscopic projection,  the "ortho-stereo seat"  is directly in front of the screen at O mm from it where:

O = F W/ w

F = Focal length of the camera
W = Width of the screen
w = width of the slide (before it was masked down for stereo window)

If a telephoto lens was used for the stereo pair, the ortho stereo seat moves away from the screen.
In front of the ortho stereo seat the stereo effect is "squeezed" while further away it is "stretched". 
Out to the side there is deviation shear or worse, loss of brightness.

Viewing stereo by projection is never as as satisfactory as a good viewer, especially for audience members nowhere near the ortho-stereo seat. (Ghost images, big people in front of you, problems if you tilt your head, insufficient blackout...)

Stereo with Full Size Transparencies (35mm or 6x6cm)

 This means each transparency is mounted as a separate slide. However, they should be mounted for viewing side by side in a simple hand viewer, with two non-prismatic lenses. When in this hand viewing mode, the most distant objects should once again be placed 63 to 65mm apart. Now when the stereo pair are projected, the two projectors are turned until the most distant objects are 63mm apart on the screen (and exactly the same height). Once this is done at the start of the show, correctly mounted slides will retain the correct separation. 

It is more practical to have the two projectors one above the other to get the lenses as close together as possible to reduce keystone distortion. (However, if you are presenting macro stereo performed by camera convergence, it is better to have the projectors converging too so the camera and projector keystone distortions can cancel out.)

 Standard mounting of each of the stereo pair means slides can be projected either stereo (two projectors) or mono (one projector), making a more versatile system than a dedicated stereoscopic mount allows.

There are quite a few stereoscopes available for seeing separately mounted stereo slides in 3d The simplest is two plastic 35mm viewers stuck together with tape. Many simple plastic 35mm viewers are fixed focus for closer than infinity, which is not good when the eyes are set parallel for seeing the stereo pair. Parallel eyes expect to focus at infinity. 

Digital projection with liquid crystal glasses is not discussed here - this page is for amateurs with limited finances. When digital projectors fall in price, and work at high refresh rates, that is probably how we will all see stereo.


Setting up prints for stereo viewing in a viewer using magnification, non-prismatic lenses also demands the distant points be separated by 6.3cm. Background separation  can be increased to the full inter-ocular distance (6.5cm) if there is a smooth transition from near to far. But if a near object is in front of a distant background, with nothing at intermediate distances, the jump in separation from near to far can be hard to achieve for some people. 

Experts can diverge to at least 8cm for free viewing of Holmes card pairs, but only from 30cm. If they are looking from the close position allowed by magnifying viewers, precious few can fuse 8cm separations. The viewer is needed even by experts to see Holmes cards at full size.

Separation more than the inter-ocular distance is fine when prism lenses are used (as in a Holmes viewer), depending on the prism power available. 

Holmes Cards

The classical Holmes card is 3.5 by 7 inches (89 x 178mm)
Each picture is 3 inches square. (76.5mm).
Actual measurements by the author on antique Holmes cards shows not all followed this rule. "Rose Stereographs" of New Zealand were sometimes 87.5 by 73mm on a 4 by 7 inch card (101 x 178mm)

Different Holmes stereoscopes have different degrees of lens divergence. You can check this by taking the stereoscope out in the sun and using it as a "burning glass". When the sun image is smallest measure:
 a)  how far it is from the lens (viewer focal length) 
 b) measure the distance between the two images. This distance is the correct infinity separation for your particular viewer. 

Usually the near point deviation from the window is zero, giving a separation of 3 inches (76.5mm)
Infinity deviation from the window is usually 5mm or less, giving a total separation of 76.5 + 5 = 81.5mm.
The total deviation can be increased above 5mm by having near objects poking forwards through the window, which needs special handling as described already. 
Experiments by the author found 10mm total deviation could be viewed by most people, but only if infinity deviation was kept at or below 5mm relative to the frame. Splitting the deviation into 5mm positive and 5mm negative is the only sensible way to include objects both at infinity and closer than 2 meters, when using a 35mm focal length lens. This split technique can work very well on Holmes cards, especially if combined with tricks to reduce edge stereo failure.


Modern commercial postcards come in various sizes. 

5 x 7 inch is a common larger postcard, which is ideal for a modern Holmes card. 

Thorpe standard

A modern standard for Holmes cards is described here, along with advice for cutting the pairs before pasting onto a card.

Lots more tips on how to cut and paste Holmes Card prints here 

Eat a Kareena,. Abstract 3D Astronomy by John Wattie

A 6x4 stereoscopic card. 

This reproduction is JPEG compressed with much reduced resolution compared with the original card.

Designed for viewing in a Holmes stereoscope, it uses parallel stereo (U).

This is a stereoscopic abstraction by the author, rather remote from reality, although based on a Southern Hemisphere Nebula just West of the Southern Cross. Astronomical stereoscopy outside the solar system is impossible: even  the earth's orbit is not long enough as a stereo base. However stereoscopic machinations such as this are a load of fun (and much more spectacular at full resolution.)

adapted from a spectacular mosaic image taken by Nathan Smith, University of Minnesota/NOAO/AURA/NSF. He used narrow band filters to detect single emission lines from gases, then coloured the combined images arbitrarily. His colours have been changed here.

4x6 Holmes stereo cards are available commercially. along with a simple viewer.


A  5 by 7 stereo card, by the author, is illustrated here at reduced size.

Because of all these precautions, beginners have found it easy to view, despite the considerable depth.  In the stereoscope, people think they are seeing a rectangular picture and only experts realise the subtle tricks  used to make viewing easy.