STEREO PHOTOGRAPHY

By Bill Dudley

This article appeared in Issue  No.25 (April 1982). Many CRC members will have never read it and with some renewed interest in 3-D work trust that most of you will appreciate its re-publication. Bill hasn`t been a CRC member for some years and contact has been lost with him. However Copyright remains with him. 

The first stereoscope was invented by Professor Wheatstone over a hundred years and since then various improvements have been made to try to perfect the viewing of the three dimensional pictures. The subject of showing colour transparencies on a screen in full-relief has intrigued me for a long time, and so I decided to have a go at it to find out what results was available. There are several ways of viewing stereo pictures and I was first of all attracted to the Polarised Light method as I already had the two necessary projectors. However having succeeded in obtaining satisfactory 3-D pictures on a screen there proved to be several disadvantages in this method. I therefore studied the subject further with the aid of an excellent little booklet published by Mr. H. W. English of the firm of the same name to whom I am indebted for permission to use extracts from it. I was fascinated to learn that it is actually possible to view stereo pictures with the unaided eye. Later in this series I will be explaining both the aforementioned methods, before doing so however I will discuss the method of taking stereo pictures and how the human eye functions. 

TAKING STEREO SLIDES 

This is perhaps the easiest part of the exersize and it can be done with no special equipment at all, but a simple devise for the camera is very helpful. Your usual camera (any type) can be used and it is only necessary to take two shots of the subject with the position changed between shots. The distance between the two camera positions varies with the focal length of the lens being used and table A gives the theoretical distance for a selection of focal lengths. have found that some variations from the figures can be tolerated, the longer the focal length the more the variation, and would advise anyone who is contemplating stereo work to carry out their own experiments for the first efforts until a satisfactory figure is found. 

I use a simple devise which fits onto the tripod as shown in Fig 1. but any solid flat surface will suffice if available. There are other more sophisticated means available for stereo work, i.e. Special Stereo cameras, Beam splitters for attaching to standard cameras etc, and these are essential if the subject contains rapid movement such as sporting activities. However for most types of picture, in particular landscapes, the results obtained are equally good with your own camera and slide. 

The  sequence for obtaining a stereo pair of slides is to take the first shot with the camera at the Right Hand end of the slide and

then (A) Move on the film (B) Slide the camera to the pre-determined Left Hand position and (C) Take the second shot as soon as possible in case the light changes. In the case of transparencies when the film is developed the stereo pair will be in the correct sequence, Left and Right for viewing or projecting by suitable  equipment. However I find

 it better to cut the pair and mount them singly (Marked Left & Right) as both my viewer and projectors take only single frames. So in my case it does not really matter which shot is taken first as long as I know which is left and right. It is therefore advisable to keep to a set routine if a number of shots are contemplated. You will appreciate i`m sure from the fore going that there is nothing difficult about the taking of stereo pictures. In the next Newsletter I shall be talking about the Human Eye and it`s functions in relation to viewing Stereoscopically. 

THE HUMAN EYE 

Fig  2  shows the human eye which can be likened to a camera with the eyeball as the camera lens. The Iris as the diaphram and the Retina as the film. In addition the human eye has an Optical nerve by means of which the signals received on the retina are conveyed to the brain.

The eye lens throws an image on to the retina exactly the same way the camera lens forms an image on the film. The retina however, is not equally sensitive over all it`s area, the most sensitive part being a small area in the centre known as the Yellow Spot, and it is directed at (Known as the Point Of Regard) the image will fall on to the Yellow Spot.

Referring now to Fig Three, it can be seen that the object A. at the point of Regard being directly in line with the Optical Axis of the eye, will be focused by the eye lens onto the Yellow Spot in the centre of the retina and can be seen in minute detail. At the same time the image of the point of B. will fall onto the retina, away from the yellow spot, which is less sensitive to detail and this object will be seen only indistinctly.

When we look at an object with both eyes they will move co-ordinately so that they have a common Point of Regard and an image at this point falls on to the yellow spot of each eye. In addition the impressions seen by the remainder of the retina in each eye are separately conveyed by means of the Optical Nerves to the brain where they are Analysed, Combined and finally interpreted as one complete picture.

Fig 4. now illustrates in a little more detail how the eye functions. In (1) A is the Point of Regard, the image of which falls o to the centre of each retina (or Yellow Spot) and is seen as one object. The point B. however is some distance nearer the eyes than point A. so the image of point B. will fall to the left of the Yellow Spot in the Left eye, and to the right of the Yellow Spot in the Right eye, giving the effect of two images nearer than the Point of Regard A. If we now make B. the Point of Regard Fig 4 (11), the situation will be reversed.

The image of B. will fall onto the Yellow Spot of each eye and thus be seen as one while the image of A. will, now fall to the Right of the Yellow Spot and to the Left of the Right eye yellow spot, which means that point A. will now be seen double and further away than Point of Regard B. This is very easily demonstrated by holding up the forefinger of each hand in front of your eyes,one about 10" away and the other about 20" away. If you first focus your eyes on the further finger the nearer one will appear double and indistinct. Then change your focus to the nearer finger and the further one will appear double and fainter. make particular note of the last statement for this demonstrates the art of viewing Stereoscopically unaided and I will return to this when discussing the method of  unaided viewing.

Here I must apologise to everyone who finds the above explanation  a little hard to understand but if you go over it several times with the diagrams to hand it will become clearer. However there are always some people who cannot see stereoscopically and to them I recommend the "Polarised Light Method" which I will be discussing later.

VIEWING BY THE POLARISED LIGHT METHOD

Having arrived at the situation where we have stereo pairs of slides for viewing by projection we will first consider the Polarised Light Method.

The idea behind this is  to project the two slides on to a screen in such a way that they are superimposed but each can be seen by a different eye. This is achieved as shown diagramically in FIG 6 by placing Polarising Filters over the two projector lenses so that the light from the Left hand projector is Horizontally Polarised and that from the Right hand projector Vertically Polarised.

These two pictures are projected on to the screen slightly out of register and will appear blurred to the naked eye. I will mention at this point that the screen`s surface must be metalised or silvered in order not to disturb the Polarising  effect. 

In order to view the combined pictures stereoscopically it is necessary for the viewer to wear Polarised Spectacles with the lenses Phased identically to the projectors, i.e. Left and Horizontal and Right and Vertical. This means that the Left eye sees only the Left-hand picture and the Right eye the Right-hand picture.

As stated previously the brain combines the two images to give one 3-D scene. Suitable Polarising Filters are available in sheet (Gelatine) form which can be easily cut to size required for both projector and spectacles.

The results achieved by the polarised light method are very good especially if you have never viewed 3-D pictures before, but there are some disadvantages. These are:- 1. A special screen is necessary. 2. Spectacles have to be worn. and 3. There is some loss of light through polarising. These factors prompted me to look for something better and after many months studying literature and doing experiments I came across the Unaided Eye Method and stereoscopic viewing was completely fascinating.

VIEWING BY THE UNAIDED EYE METHOD

This is where the fun and games begin as everyone reacts differently, some people can pick it up straight away, others may take quite a while, but get here in the end, while a few people cannot seem to master it at all.

Two projectors are necessary as for Polarised Light method and one screen with any type of surface. This screen however should be large enough to accept two projected pictures Side by Side. There should be a small space between the two pictures and  whereas you cannot view stereo slides by the polarised light method without spectacles, you can view one of the two mono pictures exactly as in normal projection if it should happen that you cannot "See" stereoscopically.

There are various ways of achieving the stereoscopic "Gaze" as it is called and sometime ago I sent out test cards with newsletters for this purpose. Since then I have come to the conclusion that the majority of people can pick it up straight away when actually projecting slides.

The viewer should position themselves as near Central as possible and some distance away from the screen (as a rough guide behind the projectors) then holding one hand up, place the tip of the fore finger so that it is central in the line of view and about 10 inches from your eyes. Focus both eyes on the finger tip, and at he same time take in the two projected picturers on the screen. The next step is to shift your focus (but not your gaze) from your finger tip to the screen. As you do this you will observe the black line dividing the two pictures separate into two black lines and the picture which emerges between these lines is in full Stereo Relief.

One you obtain this effect it can be held quite easily and you can also recapture it again should it be lost. The process is analogous to riding a bicycle - once you can do it, you can always do it  again.

You may notice a more indistinct image on either side of the main stereo picture. If this is annoying, two pieces of black card can be used like blinkers on either side of the face to shield them off. As a matter of fact these two pieces of card are another excellent aid to viewing in 3-D if they are positioned so that your Left eye can only see the Right-hand picture and your Right eye the Left-hand picture.

I have now come to the end of this article and I hope that it will stimulate others to try their hand at it.

Referances:-
(1)  "An Introduction To Three Dimensional Photography" by J. A. Buck Publications, 223. Oak Road, Rivenhall, Essex.
(2) "Stereoscopic Representations Of Three Dimensional Scenes" by W. H. English, 464. Rayleigh Road, Brentwood, Essex.

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All at the incredible price of £129.95 plus P&P, from:- The Widescreen Centre, at48. Dorset Street, London W1H 3FH. Phone 0171-9352580 or 18. Lady Bay Road, West Bridgford, Nottingham. Phone 0115-9455459. 3-D Hand Viewers also available.

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