Stereoscopic Photography

              Viewing stereo pairs without fancy equipment

by John Wattie

The two pictures are commonly seen in some kind of 3D viewer

Experts in stereoscopy do not need a viewer. They can fuse a stereo pair if the pictures are placed side by side, lined up horizontally.

Most people are familiar with computer generated 3D images, seen without a viewer, as they are widely published in books and posters. (The "Magic Eye" books, for example.)

Another trick replicates head movements to generate 3D using an oscillating image ("wobble stereo" )

To escape from frames, press here

Stereo pairs are fused without optical aid in two ways, called X (cross eye) and U (parallel).

X view: Big images are fused by going cross-eyed until the two pictures superimpose. Converging the eyes makes them focus close, and it is necessary to wait until the brain adjusts the focus for distant viewing again. Suddenly the pictures fuse as a 3D image. It is possible to look around the picture with the eyes locked into the correct format.

U view: Small images are seen the same way as in a 3D viewer, using U or parallel vision. The eyes are relaxed to look into the distance until the images fuse, then refocused by the brain.

 Many people cannot easily make their eyes diverge from parallel because that is all they have to do to see into the far distance. 

Many people can fuse Holmes card stereo pairs without the viewer (which uses base out prism lenses to increase divergence). The pictures are 76mm apart, which defines the for-ground window. The infinity points are always separated more than the window. 83mm infinity separation is easily fused by people with 68mm inter-ocular distance, using free U viewing at 30cm viewing distance. The process is easier if the for-ground is fused first, then the distant objects follow painlessly.

If the separation is too great for your eyes, (double vision), just move further away from the computer screen and the required angle will decrease. 

[Stereo viewer:] The big advantages of a viewer for U stereo:

  1. the two magnifying glasses, which make refocusing the eyes unnecessary. 

  2. A septum separates the two images, so each eye only sees the picture it is supposed to.

  3. Some stereo viewers include prism lenses to help diverge the light path.

Bigger images can be U viewed  if mirrors or prisms are used. (E.g. the [Cazes viewer,] which uses 4 mirrors, and works very well on a computer screen.) 

[M view] is not quite free viewing, but involves an ordinary mirror or prism. It is described on a separate page.

U view on a computer screen

It is easy to set up stereo pairs for parallel viewing on a computer screen, but the result is not optimal without an optical viewer. 

  • For U view, present your face close to the computer screen, relax your eyes, slowly move back.

  • The images will fuse but are blurred because relaxed eyes focus into the distance. 

  • Keep the images fused until your brain works out how to adjust the focus.

  • You will see three pictures.

  • Now tilt your head until the three pictures are exactly aligned horizontally. (Missing this step is a potent cause for failure.)

  • Concentrate on the central picture and ignore the others. Only the central picture is in 3D.

There is no way to adjust eye focus consciously. The focus muscles are out of control and work involuntarily, the same way your intestines push dinner along in your abdomen, without conscious thought. Patience is the answer.

Computer screen resolution is limited.

A magnifying glass just shows the individual pixels, so a magnifying stereo-viewer is a waste of time on a computer. The lower magnification from a [Holmes viewer ] or a plastic lorgnette viewer is acceptable on a computer. 

Low power hobby glasses are helpful because parallel eyes are normally focused at infinity and the glasses correct that. (Weak, 1 diopter magnifying glasses in a frame - get them from The Warehouse in New Zealand.)

button.gif (1435 bytes)Press to practice U and X viewing

button.gif (1435 bytes)Press to see Komrad Cave in U stereo


Single Image Random Dot Stereograms

This is how many of us started with stereoscopic free viewing.
Look through the dotted picture until the little triangles at the top fuse.

A wine glass. From Gareth and Peter's SIRDS page.

SIRDS have the advantage that eye divergence to see the image is much less than needed for stereo pairs. It is good to start with SIRDS before progressing to more difficult images on this web site. Click on the above SIRD of a wine glass to reach Gareth Richards and Peter Chang's web site with many SIRDS and links. I chose this example because it is easier to see than some.

X view works better on computers

Radiologists who work with 3D X-rays often use cross eyed viewing.

People working with large format aerial photographs or astrophotographs may do so too.

If you can see the "Magic Eye" books in 3D, then you can fuse these cave picture by going cross-eyed.

At first, move away from the screen so it looks smaller, then your eyes will not need to cross over so far.
  • A helpful trick is to look at your finger in front of the computer screen.
  • Bring the finger closer to your face, which forces your eyes to converge, and suddenly the images on the screen will fuse.
  • They will be out of focus, if you are young.
  • Once the images fuse, just sit quietly, until your brain solves the problem and adjusts your eye focus.
    • If you are old and cannot focus close, then you have an advantage, because the fused computer pictures will probably be in focus (but your finger will not be).
    •  Bifocal or reading spectacles help  - just use the close up part of the lenses and move back and forth until the fused 3D images are in focus.
  • It helps to concentrate on one feature in the picture and then gradually the rest of the image comes into 3D. A straw stalactites in cave stereo is ideal for this.
  • There will be four images at first, then three  as fusion begins. Concentrate on the middle image of the three.
  • As described for U stereo, be sure to tilt your head until the multiple images are exactly aligned horizontally. If they are not lined up, one eye has to look up or down and while that is possible, it is difficult for a beginner.
  • An even better trick is the Elliot viewer


The Elliot viewer comes from an Englishman working in the 1830's. There is controversy about Wheatstone being the first to discover stereoscopic pictures. Some authorities say  artists had already developed stereoscopic pictures, which they viewed cross-eyed, later helped with this viewer.

As early as 1584, Leonardo da Vinci, one of the great scientific artists, studied the perception of depth.

{ "Binocular drawings were made by Giovanni Battista della Porta (1538-1615), while Jacopo Chimenti da Empoli (1554-1640) produced drawings side by side which clearly indicated his understanding of binocular vision."}

The Elliot viewer could be improved with base in prism lenses, better still concave (negative) prismatic lenses. These should allow the eyes to remain parallel and force them to focus on the screen rather than the aperture. The author has not bothered since he does not need any viewer.

If you want to set this up, the light deviation in prism lenses is often quoted by opticians as "prism diopters".  

Prism diopter = tan(light deviation)/0.01

Suitable prisms are sold by Rolyn Optics Company

A piece of cardboard with a square aperture cut in it facilitates cross-eye viewing.

  1. The aperture's width should be less than the distance between your eyes: 4cm works well. 
  2. If you prefer, make an Elliott viewer out of a black cardboard box, with two holes for your eyes, but that is not really necessary.
  3. Hold the card about half way to the computer screen. 
  4. By looking with each eye in turn, adjust the aperture's alignment and distance until the left eye sees the right hand picture and the right sees the left. 
  5. Now with both eyes open, look through the hole and as long as the card remains still, you can't avoid seeing the stereo pair correctly for cross-eye vision.
  6. Patiently wait until the convergence is exact and the focus corrects itself.
  7.  Suddenly you will lock on and see in stereo. 
Mirror viewer with convergence control

This shows both parallel and cross eye stereo pictures by adjusting the outer mirrors of a 4 mirror Wheatstone system.


Half-Wave Plate

Warning: the author has not tried this. I would rather spend money on a Wheatstone mirror viewer, such as the adjustable version linked above, than on a sheet of polarising plastic. My last sheet was cut up to make spectacles for projection stereo!

  • Half-wave plates cost heaps, but it so happens Japanese engineers have found:     
       {  cellophane is a half wave plate.}
  • They say a half wave plate rotates polarised light 90 degrees.
  • According to the above link, some portable computer screens send out polarised light ( unfortunately only partially polarised on mine). However, you can arrange for any computer screen to emit polarised light.

(Polarise is polarize in USA).

  1. Place a sheet of polarising film over the whole computer screen.
  2. Place a half wave plate (cellophane) over half the polarising sheet (or one of the stereo pair of images).
  3. Now view the computer screen through a pair of polarising filters, rotated so that each eye sees the opposite half of the screen. You can set this up for either  X or U stereo by rotating each polarising filter correctly. 
  4. Now each eye only sees the image it is supposed to.

You will still have to use cross or parallel eye viewing, but at least you no longer see the confusing wrong image. There is the disadvantage that the picture is much dimmer, since it is coming through two polarising filters. 

Left Eye  

Right Eye                   

       Left Eye

3dl.gif (2197 bytes) 3dr.gif (2207 bytes)

3dl.gif (2197 bytes)

This is an easy stereoscopic image, to practice  free viewing.

The two left images are for U stereo and the two right images are for X stereo.

When viewed stereoscopically as labeled, the lowest, green 3D is in front. It  looks smaller than the others, but all 3D's are actually the same size.  We expect close objects to look bigger. If a close object is the same size as a distant one, the brain decides it must be small. If you view the left images by cross eyes  instead of parallel, the green 3D is behind the others. Viewing a stereo pair with the wrong eyes produces [pseudo-stereoscopy.]

If you are having trouble, try these simpler pairs of shells

Start with the small separation and build up to the bigger.

The first picture is the famous "wallpaper illusion" which many people find the simplest 3D impression

Practice both U stereo and X stereo: soon you will be able to swap rapidly between the two methods.

Wallpaper illusion





shell shell         shellshell





Stereo1.jpg (72508 bytes) Half size stereo image of Komrad Cave


View this New Zealand cave in X (cross-eye) stereo.

This is a small picture, which is easier to see in 3D than the 
full size version
Viewed in U (parallel) stereo, the result is not good, 
called [pseudo-stereoscopy.]

Press here for true Parallel eye stereo of Komrad cave.



View of Komrad cave  in X stereo.                        View of  Komrad cave in X stereo.

Go to smaller, easier  version.                                                 Go to  smaller, easier version. 

Go to U stereo  version                                                             Go to  U stereo version

Komrad Cave: the middle two pictures are in U stereo.

An X Stereo pair can be set up for U stereo viewing by placing two of them side by side.

The centre two pictures are then suitable for parallel viewing.

Right Left Right Left

3 sets of increasing size are available here:

The top pictures are small to avoid diverging eyes, which most people cannot handle.  The middle set need slightly diverging eyes or a Cazes viewer. (You may need to pan across to the middle pair of pictures - use the arrow keys or the horizontal scroll bar.)
  • The lowest set are for experts, or a Cazes viewer.
  • The Pokesope viewer or
  •  low power diverging prism lenses  also work with big U stereo pictures


stereo1.jpg (72508 bytes) stereo1.jpg (72508 bytes)


stereo1.jpg (72508 bytes) stereo1.jpg (72508 bytes)


stereo1.jpg (72508 bytes)   stereo1.jpg (72508 bytes)


You can always move back from the computer screen to reduce the required eye deviation. This trick works with both U and X stereo pairs. The cost is reduced spatial resolution, because the images are distant.

Images on this web site can be [converted to other stereo formats] using easily downloaded, free programs, so do not despair if you can only see 3D with [red/green glasses.]


Go to caves in stereo,
(includes anaglyph).

 3D contents page

Stereo Picture Gallery

Now you are an expert in cross eye 3D viewing, go the the Mars images in cross eye format here:

Escape from 3D:  New Zealand Images   

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