Viewing Stereocards.

There are many ways of viewing stereoscopic images. Only two really apply to traditional stereocards; these are 1) Using a viewer with two eyepieces (lenses and/or prisms), and 2) Freeviewing.

In order to generate a 3-dimensional view, the left image of the stereo pair must be viewed by the left eye, and superimposed on the right image, seen by the right eye.

1) Using a Viewer.

The most common sort of viewer is the 'Holmes' viewer, in which lenses direct the view so that the left eye sees the left image, and the right eye sees the right image, and the left and right views are superimposed.

holmes.gif (2183 bytes)

The Holmes viewer is named after Oliver Wendell Holmes. It is hardly ideal for viewing images on a monitor, but it does a good job on monochrome stereoviews. These viewers were made in very large numbers between 1850 and 1920, but are only one method of viewing stereoscopic images.

testcard.gif (2518 bytes)

2a) Freeviewing.

Freeviewing is the most obvious alternative to using a viewer, and is more appropriate for browsing stereo images on a computer monitor. The aim here is to overlay the left and right image by directing the eyes. As stereocards always have the left view on the left, and the right view on the right, 'parallel' viewing is appropriate, with each eye looking at the view in front of it. The eyes are therefore directed as if they are looking at an object in the far distance, but the image is not at a great distance, so the eyes need to focus more closely than normal for that convergence.... so short-sighted people tend to find parallel viewing (also known as wall-eyed viewing) rather more straightforward than long-sighted people.


Normal Viewing - both eyes fixate (converged on) the same point. The fields of view of the two eyes overlap.	Parallel Stereoscopic Freeviewing: Eyes pointed as if looking into the distance. They fixate similar points on the left and right views presented to them, which then fuse into a single 3D percept.


This is a representation of what you see when successfully freeviewing. The 3D percept is in the middle, flanked on either side by a ghostly image which is only seen in the left or right eye respectively. Try it on the image above this section.

The eyes don't diverge easily, so you won't want to try this with stereo pairs whose images are centered further apart than your eye separation!

2b) More Freeviewing

Still having trouble? You may find the explanations via the links page more helpful, but here are some methods for actually achieving this freeviewing thing.

Did you see those single image stereogram things? well it's like that, but with real images.
Make sure the stereo pair and your head are exactly level.
Bring the left and right image into register in one of the following ways:
- Looking at the top of the card, either relax your eyes and stare into the distance until the images separate. Encourage them apart until the two images become three, as in the picture above.
- or Look over the card into the actual distance, and see the image pairs 'snap' into register just below where you are fixating.
- or Use something transparent, flat and shiny (glass, Perspex). Place that in front of the card or monitor, and look at the reflection of your face in it. Move your attention to the stereo image, and you might be there!
- or Use 'Blobs', either on or above the image. They might be easier to register than the images themselves.
Bring the combined image into focus (remember, you looked beyond the original image to get the convergence right). This should happen if you wait a while (5-10secs) if it hasn't happened already. This bit may be difficult if you are long sighted. Sorry.
Say "Wow!". Stereonuts like that ;*)

This page is ŠInsect 2001-2002. If you want to use the material for non-commercial applications I would be honoured, but please a) let me know b) acknowledge the source c) link to this webpage. Thanks.

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