Stereoscopic Pictures

(and how to view them)

by Martin J. Powell

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^"The eyes are horizontally separated by about 6.3 cm (2.5 in) and so receive slightly different views. This can be seen quite clearly if first one eye and then the other is held open. A nearby object will appear to shift sideways in relation to more distant objects. This slight difference between the images is known as disparity.  It gives us perception of depth by stereoscopic vision.

Disparity is only part of what gives us depth perception. In addition, the eyes pivot inwards for viewing nearby objects; the distance is signalled to the brain by this angle of convergence.

Stereo vision in humans only functions for comparatively near objects, beyond which the difference between the two images becomes too small for the brain to compute distance information. Humans are effectively one-eyed for distances greater than about 100 metres.

If the difference between the viewing positions of an object received by the eyes are so great that the corresponding features fall outside the range for fusion, a curious effect occurs.  Parts of each eye's picture are successively combined and rejected, in a process known as retinal rivalry.  This rivalry also occurs where different colours are received by the two eyes, although a fusion into mixture colours is achieved by the brain."

- Extracts taken from 'Eye and Brain: The Psychology of Seeing' by R L Gregory (Weidenfeld & Nicolson, London, 1977)

How to see the pictures stereoscopically

The following photographs were each taken from two slightly different positions - roughly 1 ft to 3 ft (0.3 m to 0.9 m) apart. Since this is a considerably greater distance apart than the eyes, the stereoscopic effect is no doubt exaggerated. Consequently, the depth at which stereoscopic vision is possible in these pictures goes beyond that which we would achieve in reality (this is known as hyperstereoscopy).

The pictures - commonly called stereo pairs, stereographs or side-by-side stereo - make use of the parallel viewing method (also known as wall-eyed or wide-eyed viewing). Seeing them stereoscopically requires concentration and may take a while to master.

Adopt a comfortable viewing distance from the monitor so that your eyes are not straining in any way (in these pictures the stereoscopic effect is not dependent upon distance, since the two images are fixed in space and present the same information to the eyes regardless of the viewing distance). You may however need to move away from the screen to successfully merge the larger-sized images.

Whilst focussing on the pictures, allow your eyes to relax so that you begin to see a double image of the picture (this simply means that your eyes are no longer converging on the picture, but are looking parallel to each other, as they do in the distant vision position). In effect, you should see FOUR individual pictures - two generated by each eye.

You may try tilting your head slightly to one side when you relax your eyes. The double images should be easier to see, slightly overlapping. Concentrate on the picture on the RIGHT (this is the picture in which the stereoscopic effect will be seen). Whilst your eyes are in the relaxed state you should become aware of an image very close to it (which is in fact the left-hand picture seen by the other eye). By carefully adjusting the relaxed state of the eyes, you should attempt to merge the left picture with the right picture. If you have your head slightly tilted, slowly return it to the upright position and attempt to merge the images.

Your focus should remain on the screen and the pictures should not appear blurred. Your head MUST be level with the picture for the effect to work.

When the two pictures have become merged the brain will register the images as it would in reality, and the stereoscopic effect should pop into view. Distance in the picture should become very apparent and a surprising amount of detail will be seen.

The following photographs were taken in the Dartmoor region of Devon county, in the UK:

This prehistoric tomb dates from around 3,500 BC and is located a few miles outside Cardiff in South Wales:

The following photographs were taken in and around my local churchyard:

... and this parish church stands in a quiet village on the England-Wales border:

The following picture is best viewed in dim lighting conditions (or better still, in darkness):

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Copyright  Martin J Powell  2001, with minor text revisions Feb 2006, Apr 2015