Principles of stereoscopic photography
using an ordinary camera

Two pictures of the same object are needed, taken from two points, just as our eyes are horizontally separated.

TOPICS

1. Camera methods for stereoscopic pictures

2. The easiest, Cha Cha way for anybody

3. Stereo base, Stereo Parallax: how far to separate the cameras.

   • Isomorphic stereoscopy

   • 2° rule for stereoscopes

   • Stereo Parallax

   • Bercovitz   and Di Marzio formulae

4. Camera convergence: Complications include:
         keystone effect, crossing backgrounds, segmental stereo failure...

5. Masking for window effect

6. Eye poking for edge stereo failure (also links to the "falling frame")

7. Macro-stereoscopy: a summary of close up techniques, (not all are good!)

8. Bad technique: by not following the rules on this page, a poor stereo picture is made.

9. Better technique: how the Pohutukawa macro stereo was made.

10. Slide and print mounting

11. Mapping 3D space photographically. More philosophy than photography.

     Contents using frames page

Beginners

Until you are an expert, the following may seem very obscure. No worries -  you don't have to know much for a heap of fun! The contents frame to the left allows you to jump over the boring bits. On purpose, easy stuff is mixed up with erudite stuff here, which may just mean nobody is happy, but at least shared misery is fair to all?

Actually you can take 3d pictures without knowing much at all. 
Beginners go direct to here.

Real experts, especially the mathematical ones, will find this web site too easy, or wrong. Protests can be directed to the
[guest book for nzphoto.tripod.com/].

The only differences between the two pictures of  the stereo pair should be:

1. the shift is horizontal (called X parallax in remote imaging circles).

2. no changes occur in the vertical direction. (Y parallax is constant in the two pictures).

3. the stereo angular parallax between nearest and furthest object is around 2 degrees when seen in the stereoscope. 

4. vertical edges of the frames are masked correctly for stereo window.

Click here for example

 Any other difference spoils or even ruins the stereoscopy. 

Factors commonly seen spoiling the 3d include:

1. Tilting of one frame relative to the other, one component of Y parallax error.
2. Different magnification in part or all of the pictures (which changes things in the vertical direction, when only horizontal changes are acceptable.)
3. Objects at infinity have too much parallax: more than the inter-ocular distance, and so cannot be fused.
4. Incorrect virtual stereo window. "Floating frames" or objects in front of the window frame.
5. Excessive or insufficient stereo shift for the viewing method used. Related to this are distortions and deformations of the image. For example a circle becomes mishapen and its centre seems displaced away from the viewer. "Squeeze" "stretch" and "bulge" are terms for various deformations. Deformation from a widee angle lens used too close to the object is a parallax error and not always a stereo error
6. Bottom or top edges not at the same level in the two pictures. 
   1. which means one picture is higher than the other, or 
   2. one is tilted relative to the other, or
   3. one is bigger than the other, or
   4. one camera was aimed higher than the other.
      (The last is  partially fixed in Photoshop by cropping and the size problem is only partially correctable by image transform, but tilt is readily fixed).
7. Retinal rivalry occurs when an object in one window is different from an object in the other. For example, a moving cloud. The brain cannot decide what is correct and often flips between the two versions, which is disturbing.
8. Out of focus parts are objectionable in stereo but can still be fused and may become artistic if you are inclined that way, so this is a minor "fault".
9. Noise (grain, electronic noise, JPEG heavy compression or dust and scratches)  cannot be fused and is not good. It is suggested that a small amount of blur should be applied to noisy images to overcome this problem. A slightly blurred picture also has the advantage of better JPEG compression. Often the noise is only in one colour channel (frequently blue) and so you can blur just that channel.
10. Differences in colour and density between the pairs are not ideal but are not disastrous. Density or contrast differences can be misinterpreted as lustre or may cause retinal rivalry.
11. Edge and segmental stereo failure are discussed in detail later...

People spend a lot of time looking at a good stereo pair, because there is so much more information than on a single flat picture. This prolonged study means poor photographic technique becomes disturbing. A "simple snapshot" that is just glanced at in a home album is unsatisfactory in stereo.

Camera methods for 3D

A stereo camera is the obvious method - too obvious for this web site! 
The best stereo camera is probably the RBT: { information here }. The RBT is two high quality 35mm cameras joined together. With the RBT macro attachment it will also take close-up stereo. There is no way to vary the separation, apart from the macro system. RBT is unfortunately too expensive for the author!

An SLR camera fitted with a { "Lens in a Cap" } split image system. This is great for family snaps but a bit limited for serious stereo. Unfortunately the stereo window is wrong: infinity instead of 2 meters away. The stereo base is small, but adequate for near-by portraits. The two images are slightly different in size, corrected in Photoshop by size transform.

Two ordinary cameras with the same focal length lens will do the trick - they do not even have to be from the same manufacturer. 

• The cameras are mounted together on a stereo bar, which may just be a flash bracket or an {elegant device} allowing vertical or horizontal formats. 
• Old fashioned cameras without zoom lenses are best - 
  • it is very hard to get precisely the same size picture with two indefinite zoom settings, but a couple of dedicated 35mm lenses, for example, are likely to have  precisely equal focal lengths.
  • Different batch lenses can have different colours. Different even from the same manufacturer (including Nikon). Check by looking through them at white paper. (Colour differences can be corrected in Photoshop later, but it is easier if you do not have to.)
  • Sony digital cameras can operate with zoom lenses, linked by a Rob Crocket electronic device.
• A double cable release will fire both cameras almost simultaneously. 
  • You can release the shutters nearly simultaneously with one hand on each button - the double release is not essential, as you start out anyway.
  • Two electrical shutter releases, are usually nearer to simultaneous than mechanical cables.
  • Sony digital cameras linked with the Lanc Shepherd will fire nearly simultaneously if they are running with the clock timing in synchrony.
• Flash is more difficult when exactly simultaneous shutters are uncertain. The flash will fire correctly for the camera it is attached to, but the other may not have its shutter open. 
  • A slow shutter speed and wiring the flash sockets in series may work, since the flash will not fire until both cameras have their contacts closed. Often the camera metal case is in the flash circuit and so you cannot get the flash in series if the cameras are on the same metal bar.
  • Rob Crockett's Land Shepherd Pro will fire an external flash correctly using paired Sony digital cameras.

Any, single  ordinary camera.  

1. If the subject stays still, stereoscopy is possible with two separated pictures from one camera. Cha cha method: lean to the left:   take a picture lean to the right: take the "same" picture  LEFT click RIGHT click (Cha-cha) Cha Cha Cha method: Lean left:             take a picture Lean right:           take the picture Step to the right: take the picture LEFT one RIGHT two STEP three (Cha-Cha-Cha)    Three stereo separations result: 1-2 short, 2--3, longer, 1---3 longest.    When the pictures come back, choose the best pair. One - Two -- Three (Cha Cha Cha)  is a great method for beginners. Now jump over the hard theoretical stuff and go to here for practical advice!

Obsessional method:
slide the camera along a horizontal bar between the two pictures,
or use a variety of appliances of varying complexity for producing a rapid shift of one camera. You can invent your own methods (like the author) or get commercial systems. 

Precautions:

1. Take one image and then move sideways for another picture.
2. The two positions should be on the same horizontal line. 
3. Work as quickly as possible, to avoid things (like clouds) moving between shots.
4. Set the camera on "burst mode" and rapidly shift sideways while it is firing like a machine-gun. Surprisingly good results are obtainable on people who are not moving much.
5. Aim at exactly the same point each time - preferably in the foreground to simplify "window effect". (Aim on a background object to avoid toe-in once you are an "expert")
6. Never tilt the camera. (It can be corrected later while mounting the prints, but is a big pain.)
7. If you are taking prints rather than slides, hold the camera vertical - it will be easier to view the prints later. 
   • Vertical 4x6 prints can be seen with a simple lorgnette viewer.
   • Unless you have an over and under viewer, which is designed for landscape format, 
   • or you are good at X stereo.
8. It is better if the two pictures have the same exposure and contrast, but not essential for 3D effect. In fact it is possible to expose one for shadows and the other for highlights and (almost) get away with it.
9. To increase depth of field, focus one camera slightly closer than the other. When the brain fuses the stereo images it will present the correctly focussed parts and suppress the blurred parts automatically. This means stereo gives you a depth of focus bonus not supplied by flat photography.
10. The films should be from the same batch and processed at the same time to avoid colour shifts, but you can usually get away with breaking this law too.

The next section is boring, so jump over it if you are in a rush.

Special methods

How big should the stereo shift be?

The distance between the camera positions is called the stereo base and numerous mathematical methods are available for defining it.

The simplest is the "one in thirty rule". The stereo base should be 1/30 of the distance to the nearest object.

• The two viewpoints should be on the same horizontal line and not too far apart, or the brain cannot fuse the stereo pair.
• You might think the cameras should be the same distance apart as human eyes (65mm adult average) but that is not essential for a 3D impression. (See later for "orthostereoscopy"
• Excessive depth of planes in the picture  becomes a problem when mounting slides for projection or for a stereoscope. There are various mathematical formulae to allow for this, but they are not practical in the field. On this web site you will find out how to take spectacular stereo without any mathematics.
• As discussed in the physiology section, double vision is perfectly normal when viewing scenery with great depth, but neophytes to stereoscopy are not easily able to handle it. If you want your pictures acceptable to the masses, do not have excessive base shift.
  •  (Stereoscopic impression by the observer's "cyclopean eye" is actually built up in the brain from repeated corrections of diplopia as the eyes scan the scenery. This takes time and is  never instantaneous - see physiology section).
• Close-up photography and telephoto photography run into the problem of limited depth of field. It is not good for out of focus objects to show up in stereoscopic photographs as they said to spoil the 3d effect. This sets a simple practical limit: if out of focus objects are excluded from the picture, then there is less chance of suffering from excessive stereo depth.
  (Depth of focus is the traditional stereo limit in books. In the author's experience, out of focus objects are seen in 3D and if they are a significant part of the scene it is important to make sure they can be viewed; by not having them separated by more than the inter-ocular distance.)
• This web site is designed for "optical athletes" working without optical aid on a computer screen and so many of the stereograms here break the "rules".

The next section is boring for beginners, so jump over it !

Two degree rule

• If stereo reality is not the aim, the angle  between lines running from the two camera lenses to the nearest object should be about 2 degrees, as long as it is a standard lens for the camera format. In other words, the angular parallax should be a maximum of 2 degrees.
  • (Angle of horizontal view, from edge to edge of the picture, with a standard lens, is about 40 degrees)   The 2 degree rule does not work for telephoto lenses and is actually designed to work with Stereo-Realist 35mm lenses in 35mm format photography.
• This agrees with the "one in 30 rule":
    arc tan (1/30) = 1.9 degrees.
• Although often quoted as a requirement when taking a stereo pair, the 2 degree rule is fundamentally a principle to be observed when seeing the final picture. In other words the viewing system you use to see stereo is a vital part of the equation. The focal length of the stereoscope should be the same as the focal length of the camera lens. This way the angular field of view is the same for the camera and the viewer. Perspective is then shown correctly. When telephoto or wide angle lenses are used on the camera, the 1 in 30 rule breaks down.
  • (The 2 degree rule on the final pictures allows the stereo window, in front of the closest objects, to be set at 2 meters from the observer; when the most distant object in the scene is separated on the two images by the inter-ocular distance of 65mm. See later for details if this seems obscure.)
    (Some workers claim that objects more than 2 meters away are seen without double vision and that is why the 2 degree rule is good. In fact the Panum zone where diplopia stops is much smaller, about 12 arc minutes, (0.2 degree) as explained in the visual physiology section. Diplopia only stops around 18 meters away. But you have to concentrate to see that and in ordinary life it seems 2 meters away is the limit for diplopia that is annoying.)
• Bigger angles than 2 degrees can be fused, but beginners may have trouble at first because they cannot tolerate seeing double. Seeing double is perfectly normal in the real world: hold up one finger and look at a distant object, You will see two fingers every time! So objecting to diplopia (double vision) in a stereo scene is illogical. Even so, if beginners are to see your results, you had better not make them aware they are seeing double!
•  When you look at something 30cm away (close up) the parallax angle is about 12 degrees. More than 12 degrees and even experts get a pain in the eyes.
    2* arc tan (65/2/300) = 12.4 degrees)
• For stereoscopic X-rays, we often use 5 degrees (the stereo sea-shell x-ray used 7 degrees).
• If you break the 2 degree rule, you will need to use extra deep masking windows, or the "poke your eyes out technique" to get away with it.
  

Stereo parallax

• Stereo parallax is the difference in separation between near and distant objects when mounted for viewing. 
  • Stereo parallax is expressed as angular parallax or linear parallax. 
  • Angular parallax is the degree of convergence needed to fuse an object.
    An object at stereoscopic infinity has an angular parallax of zero in the real world, but this is only true on a stereoscopic pair if they have been set up properly for the individual.
  • Linear parallax is the difference in distance between homologous points on the stereo pair. If the linear parallax of an object is subtracted  from the linear parallax of an object known to be at infinity, the distance can be computed.
    Often in photogrammetry there is no object at infinity and the parallax must be scaled by actually measuring on the ground. ("Ground control" of spot heights and distances to give the plate magnification).
• The "allowable" parallax in commercial stereo photography depends on what a beginner can achieve. Experts in photogrammetry, who use stereoscopy all the time for measuring heights in aerial photographs, or remote sensing satellite images, can handle a much greater parallax than the average person.
• In 35mm slide format, it is best to keep linear parallax below 1.2mm.
  (That is 1.2/35, near enough to 1/30 of the image width, in landscape format)
  For a 35mm lens, (standard for many stereo cameras):
    Inverse tan 1.2/35 = 2 degrees.
  However for a 50mm lens:
    Inverse tan 1.2/50 = 1.4 degrees
• In 6x6cm format, the linear parallax should not exceed 2.8mm
  For an 80mm lens ("standard" for Hasselblad and standard for a 6x12 paired stereo format viewer).
  Inverse tan 2.8/80 = 2 degrees.

• People  look at pictures, and computer screens, from a "comfortable distance". 
• Observers move back from big pictures and closer to small pictures until they are at the preferred size. So the absolute size is not of great moment, the angles are what matter.
  • Makers of electro-optical shutters for computer games prefer a  maximum parallax of 1.5 degrees. 
  • "Zero parallax" has a different meaning in computer games than in photogrammetry. When objects are in the plane of the monitor screen they are said to have "zero parallax". Objects in front of the screen have "negative parallax" while objects behind the screen have "positive parallax". This is a very confusing use of the term "parallax" and so I will put it in inverted commas.
  • Used in this sense, "zero parallax" reduces cross-talk between left and right images caused by the computer phosphors not turning off instantly. Green phosphors  have longer persistence and 3d computer artists may reduce green levels to minimise cross-talk when designing for liquid crystal glasses. 
  • Similarly, anaglyphs have bad cross-talk and this is reduced if the "parallax" is kept small and close to the screen surface. 
• If you can split the "stereo parallax" into positive and negative components, so much the better. Half the picture in CRT space and the other half in observer space allows greater stereo depth, while still keeping close to the screen surface. 
• In the case of stereo prints or transparencies, a similar advantage applies. "Zero parallax" usually defines the stereo window which in turn defines how much linear parallax infinity objects can have in the depths of picture space. If you can get half the stereo depth into air space, without ruining the window effect, the limitation on zero divergence for infinity objects allows much closer objects than 2 meters to be included in the picture. 

Note how the term "divergence" has been slipped in. Since "zero parallax" means the stereo window, it can no longer be used for the zero angular parallax of objects at infinity. So we now have total confusion of terminology depending on the book you happen to be reading:

•  angular parallax can be absolute or relative. It is measured in degrees or radians.

• absolute angular parallax of an object at stereoscopic infinity is zero.

• angular parallax is the only term that seems to have a consistent definition.

linear parallax can only be relative. It is measured in mm or inches.

• divergence is a term used by some stereo photographers and is the relative linear parallax when the linear parallax of the stereo window or computer screen is assigned the value zero. So divergence can be positive or negative.

• "parallax" as used by computer artists is similar to the term divergence as used by stereo photographers.

An interesting effect occurs if the observer moves away from the screen. The screen looks smaller,  but the stereo depth seems to increase. This is called "stretch." 

• For example, while sitting 20cm from the screen an object may seem to poke out into observer space by 4cm. Now move back to 40 cm and the object looks as if it juts 8cm out of the screen. It seems to persist at 1/5  between the observer and the screen.
• For spectacular 3d it is better to sit back. 
• For immersing yourself in the picture it is better to sit forward. 
• The picture looks  bigger from near by. But as you come closer, the parallax also looks bigger and the ocular shift needed to fuse the stereo pair increases. Ultimately the angular parallax may become too big for you to fuse and uncomfortable double vision results. Also, resolution on a computer screen is limited. If you sit too close the pixels show up.
• The ortho stereo seat in projection stereo is the place in the audience where the stereo effect looks the same as real life. It is exactly in the mid-line of the projector to screen axis and at such a distance from the screen that the angle of view of the projected picture is the same as the angle of view of the taking lens at the time of photography. Only one person can sit in the ortho stereo seat, everybody else is seeing a distortion of some kind.

This Mingimingi seedling pokes out of the picture. Try viewing it  from different distances to see if you agree with the above - not everybody is aware of stretch!  (Some browsers will not show this properly - Internet Explorer and Opera are good. The two left pictures are X stereo and the two right pictures are U stereo)

Hyperstereoscopy

• If you aim for big parallax angles when photographing objects which are clearly not close at hand, the result can be unrealistic, although spectacular.
• Too much stereo separation distorts objects by making them seem too deep, which is usually disturbing rather than satisfying. A round tree trunk becomes an oval, with the long diameter running into the picture.
• However wide separation macro stereoscopy is said to be valuable for thin objects, like coins, as it displays fine depth differences well. (Stereo magnification).
  
• There are formulae for the stereo base in hyperstereoscopy (one of the simpler ones being the camera separation should not exceed 1/30 to 1/50 of the distance to the nearest plane. (This rule only applies when infinity is included in the picture. It computes too short a stereo-base for objects with limited depth).
  • The 1/30 rule is based on a stereo window at 2 meters (65mm x 30 = 1950mm = 6.5 feet). This means when using a stereo camera whose standard focal length lenses are about the same distance apart as human eyes (65mm), and infinity objects are in the picture, you should not have foreground things closer than 2 meters away (around 7 feet). 
  • Otherwise very distant objects will be separated by too much for easy fusion when the two pictures are mounted with the stereo window set at or in front of the closest object. The limit is set by how much the observers can diverge their eyes and most beginners cannot go beyond parallel. 
  • Experts who can go beyond parallel may find that although the pictures are fused, they are suffering "stereo paralysis". The fused picture looks flat. (It will also look flat if one of the eyes "turns off" to overcome diplopia. Ocular suppression is mentioned further in the physiology section)
  • Standard stereo mounts will not work properly with stereo base of less than 1/30. Special windowing techniques are needed for extreme stereo depth.

   This means distant objects must be no further apart on the stereo pair of images than the inter-ocular distance. 
  In those viewers which use diverging or prism lenses, the allowable separation is bigger.

A Stereo bar and the traditional 1/30 rule

When two 35mm cameras with 35mm lenses are set up on a stereo bar and infinity is included in the picture: they should not photograph objects closer than:

N = B x 30
N = nearest object
B = stereo base

Example: Two 35mm cameras with 35mm lenses set 12cm apart:

>    N = 0.12 x 30 = 3.6 meters, around 12 feet = 4 walking paces.

Hyperfocal distance calculation for this set-up:
   f  =  F² / Hc
       f = f number
       F = focal length of the lens
       c = acceptable circle of confusion (usually 50/1000 for 35mm format, or one thousandth of the standard focal length for the format used)
       H = Hyperfocal distance, and H is twice the nearest distance still in acceptable focus:

    f = 35² / (3600 x 2 x 50/1000)
      = 3.4
This means: 
     use f3.5 or bigger, 
     leave focus on 7 meters 
     and have nothing closer than 4 paces.
     Which means a person standing fits nicely in the horizontal frame.

Note that 35mm lenses do not produce orthostereo unless the stereoscope also has 35mm lenses.
Strictly the 1/30 rule should be modified to:
Stereo base = 1/30(V/F)
where V is the focal length of the stereoscope and F is the focal length of the camera.
Even this will not produce "orthostereo" as discussed above, but at least the stereo pair can be mounted in a standard stereo mount.

Practical method using one camera:

For those who are obsessional, measuring callipers, or dividers, or lines on transparent plastic can be set up at :

C = .035AV/F 

C = calliper opening
A = length of your arm
.035 = tan2°
V = stereo viewer lens focal length
F = camera focal length

Then hold the callipers at arms length and measure the parallax between nearest plane and furthest plane as you move sideways - whew. 
You could always measure angles with a sextant held horizontal...

Please note that experts in stereo vision may often break these very conservative rules, at the expense of having beginners walk off in disgust when they cannot fuse the pictures. However, the experts are getting a real mind-blowing blast of depth perception and it is worth practicing just for the kicks - all achieved without drugs!

Hypostereoscopy

For extreme close-up photography (less than 30cm) most workers say the camera separation should be reduced below the 6.5cm average inter-ocular distance. This is sometimes called hypo-stereoscopy.  Most examples of excessive stereo shift are seen in macro-photography. However, there has been a recent re-think of this! A method for close-up stereo pictures, which does not involve any mathematics, is given here.

Complex formulae exist for computing stereo base. A recent versions are:
the [Bercovitz formula] and
[DiMarzio formula]. Surprisingly, the Bercovitz formula shows stereo separation for a near-by object can exceed the normal  inter-ocular distance. I was sceptical, but did an experiment, described here, and found it works. However, as they say in the advertisements for unbelievable bargains: "conditions apply". The formula also gives interesting results when applied to telephoto hyperstereosocpy, and the obsessional workers will have to check it out. Beginners, read on here.

Virtual stereo window

Click here for diagram

• Consider the left edge of a window.  The right eye looks around the frame to objects which are hidden from the left eye, behind the frame. The right eye views more of the scenery to the left than the left eye does. 
• The opposite applies to the right edge of the frame: the left eye views more than the right eye can. 
• If an object is the same distance from a frame edge on both pictures, it is exactly in the frame - the "stuck on the glass" effect.
• More information with diagrams { here }

EXAMPLE

A cocktail glass photographed in a picture frame, 
with a map in the background. 

1. The cocktail glass just behind the frame can be seen with both eyes.

2. New Zealand's South Island is partially cut off by the frame in the left eye view, but the whole island can be seen with the right eye.

3. Background portions only seen with one eye at a time have stereo failure. They are monoscopic.

Parallel eye stereo (U)	LEFT	RIGHT

Cross-eye stereo (X)	RIGHT	LEFT

Segmental Stereo failure

Any segment visible with one eye but not the other is obviously "mono" and not "stereo". 

• An object in the foreground hides a background feature for one eye but not the other. 
• This applies to the edges of the stereo virtual window frame, if it is properly set up to lie in front of the scenery. 
• For shallow scenery, this is not much of a problem, but if there is considerable depth (as there often is with macro and hyper stereoscopy), the segment with stereo failure becomes wide and somewhat disturbing.

Camera Convergence

Crossing background

•  If you do converge axes, make sure background objects do not move out of the frame or confusion results. e.g. the left eye sees a different background to the right eye and nothing in the distance can be fused by the brain.
• The crossing background is really a variation on the theme of segmental stereo failure.
• A crossing background is likely in macro-photography using convergence, so don't have a distant background, which would be out of focus anyway.
   (See the koru: macro stereo with no background objects included).
• If you wish to have the background in a macro-photograph out of focus and yet still in 3D, with no crossing over, you may have to move the background sideways between the two pictures. 
  • This is an optical bench and table-top photography technique, which can be a remarkably effective in flower photography. e.g. the background can be foliage in a pot, which may be moved without disordering it. 
  • Do not slide the background so far that it comes into the foreground plane. 
  • You will discover the claims sometimes made that "out of focus objects cannot be seen in stereo" is totally untrue. In fact the artistic effect can be gentle and pleasing.
  • This is as good a place as any to point out that remarkably good stereo can be seen if one picture is in focus and the other blurred, as sometimes happens by accident. 
  • Selective blurring can even be used on purpose to increase depth of field, by having one camera focus near and the other far. It is not a "professional" technique - obsessional photo-club judges will complain bitterly if they detect focus differences - but you can often get away with it if the individual stereo pairs are not examined separately. It is hard to know why club judges would do that, after-all the pairs are presented only for stereo viewing, but they are obsessional personalities, struggling to find something to complain about, even if it makes no sense... A slight magnification difference also results when cameras are focussed at different distances, but even that can be tolerated by all but the most pernickety. (Unfortunately, the author has "graduated" into the pedantic faction, which reduces his fun level, but he is still prepared to use this depth of field trick, which can be remarkably useful in macro photography).
• Rotating the subject and keeping the camera still for macro stereoscopy means the background is in "mono" while the subject is "stereo". This causes problems as the stereo object is in the same plane as the mono background.  Rotation completely removes the crossing background problem, but not the keystone problem.

Keystone effect

There is no doubt our eyes converge to see close up objects, but eyes are not cameras. The eye image is formed on the inside of the eye ball, which is spherical. Camera film is flat, so the geometry is different. Quite apart from geometry, the brain makes adjustments which have been learned in infancy when the visual world and the tactile world have to agree with each other.
(e.g.: Mathematical arguments about the horopter come to nothing when the horopter is actually measured by visual physiologists, who find it makes sense visually, even if the mathematics is violated.

• When a camera is turned to the right to photograph a near-by flat surface the left side, which is nearer the turned camera, is magnified. 
• If the camera is now moved sideways and turned to the left (convergence) magnification occurs again, but on the right side this time.
• If these two pictures are now set up in a stereoscope, the sides of the pictures have different magnification and the eyes have to compensate for that. 
• The keystone of a bridge is bigger at the top than at the bottom, so differential magnification is called "the keystone effect."
• Click for example

Projection stereo and keystone effect

• It does look odd when converging stereo pairs are projected (using polarised light and a metallic screen) because the pictures do not superimpose at the edges due to different magnification.
•  Keystone effect can be overcome by converging the projectors. This is not the at the same angle the camera was converged at, because allowance has to be made for the different focal length between camera and projector.  (Eric Scanlon converges his projectors and has analysed the keystone effect mathematically, since it is the method he uses for lovely close up stereo pairs of New Zealand Orchids he is famous for).
• This suggests projection stereo pairs could be set up differently to directly viewed stereo pairs, but most people (apart from Eric!) think it is best to have the mounting system identical for the two methods.

Don't converge

1. different magnification at the edges of the pictures and 
2. a crossing background. (!) 

Avoiding camera convergence is geometrically the same as using a shift lens to prevent converging verticals in architectural photography. The essential feature is keeping the film in the same plane for both pictures. 

A stereo camera only has one film plane and a fixed stereo shift, which means convergence technicalities do not occur in normal use. 

A stereo camera is boring to use and a pain for macro or hyper stereoscopy. However it excels for moving objects, synchronised flash and realistic stereo depth (if the stereo viewer has the same focal length as the camera lens). There is room for all tastes in this game!

If the pictures are cropped for presentation to make a window effect, then background segmental stereo failure is not cured by avoiding  convergence, although total cross-over may be prevented. A stereo camera with macro attachments may not have the windowing problem, but it is actually using convergence and does have key-stone effect... This web site tells you how to take close-up stereo with no keystone distortion, but it means using just one camera, or using moving stereo camera on a bar as if it were just one camera - another paradox!

Cutting pictures to make windows

Close-up stereo photography without camera convergence means the virtual stereo window for the uncut film frames is at infinity.
(Convergence is possible with a stereo camera if each frame is exposed separately, rotating the camera between frames.)

• An infinity window no good. The virtual frame must be in front of the object for proper stereo window effect. 
• The frames must cut down or masked, by slicing off the left edge of the left eye picture and right edge of the right eye picture.
   (Click here for diagram)
• If the object is very close (macro stereo), precious little of the frames are left after they have been cut! 
• However, the camera shift "should" be less than the inter-ocular distance for really close objects (to keep with the 2 degree rule), which reduces the amount of lost frame.
• If the background is well back, so much is cut away from opposite sides that edge stereo failure is inevitable, with or without camera convergence.
• With extreme magnification, some convergence may still be needed, even by the "keystone purists", especially if full width horizontal frames are desired.

"Poke your eye out" effect

• Sometimes things poking through the "window frame" are arranged on purpose, to give a fascinating illusion, but if used badly the effect is confusing.
• It is better to have objects which poke through the frame coming from the middle of the picture. Top or bottom edge is often acceptable, but coming forward from the sides is not desirable, as it spoils the "window effect". 
• Since the frame is back with the main subject, edge stereo failure is reduced.
• Have the main part of the object behind the frame: for example a plant is behind the frame, but a flower on the plant pokes forward through the frame. If everything is in front of the frame, the effect is confusing and beginners may not even be able to see the picture in 3D.
• The "falling frame" is a variation on the poke your eyes out technique.
• People focus their eyes on the computer screen. Objects in front or behind the screen must be focussed as if they are still on the screen. That is a defect of stereo photography: focus and convergence of the eyes are not working as they do in real life. An advantage of arranging the stereo window to be half way between the front and back objects is that zero parallax occurs on the computer screen and the change in eye focus (accommodation) needed to see far and near is reduced to the minimum possible.
• When stereo is projected, only those in the ortho-stereo seat (where the angle of view of the screen is the same as the angle of view of the camera) see correct depth. People well back will see a stretched depth and objects poking out from the window can become objectionable. However, experiment has shown the only people who object are stereo club judges, and the average audience just enjoys themselves.

"Eye poking technique" removes most of the edge stereo failure in macro stereoscopy. 
X stereo.
Press for Kauri tree seedling, including U stereo version.

Computer stereo terms

• "Positive parallax" means objects are behind the computer screen (in CRT space). 
  "Negative parallax" is in front of the screen (observer space). 
  "Zero parallax" is right on the screen. 
  • "ZPS" means zero parallax setting, which is used for adjusting both the stereo window and the screen plane. 
  • If stereoscopic transparencies are superimposed, objects which are single are in ZPS, objects doubled are in front or behind ZPS.
  • Setting up stereo pairs or anaglyphs starts with removing tilt, then removing any vertical parallax. followed by setting ZPS. The images are moved  until objects at ZPS are exactly superimposed. 
  • In Photoshop windowing is done by tapping the left and right arrow keys (which are then called HIT keys - Horizontal Image Translation). There must be NO vertical motion - do not touch the up and down arrow keys once vertical parallax has been removed).

• This is called parallax shear.
•  Stereoscopic shear distortion caused by the operator moving his head while remote controlling a vehicle by video stereo display could cause accidents, because things seem to move when they are actually stationary. Parallax shear is minimal at ZPS (on the screen surface) but gets worse the more objects are away from the screen surface. Positive and more especially negative parallax can cause parallax shear.

Mounting slides or prints for a stereo viewer or projector

 Stereo Home Page