To toe in or not to toe in, that's the question
 

Yes, that is quite a common question that seem to come up in this forum with a certain regularity. In reality, this question is absurd and should not be asked except on some very rare occasion when you may want to take your audience for a brief moment of spin. Under any other circumstances, the answer to that question is a resounding no.

And no, I am not saying this because of the keystoning problem usually associated with toeing in. I say that is not a problem at all because if you ever want to take your audience for a toe-in spin, you may as well add the keystone spin to the mix.

The reason why toe in is almost never a good idea has to do with how our vision works. We all know that when our eyes gaze to infinity, they gaze in parallel. That is a given that no one argues with. What most people forget is that infinity is not hundreds of millions of gazillions miles or kilometers away. As far as the eyes are concerned, infinity is quite close, so close indeed that we almost always gaze to infinity. Why? Because our eyes are only 55-65 mm apart, while the objects we are looking at are almost always meters away. One meter is 1000 mm, a gigantic distance compared to the distance between the eyes.

When calculating the angle to toe in, we need to consider each eye separately, that is as if it was some 30 mm away from some imaginary center in-between the eyes. And we need to pretend the object we are looking at is precisely in front of this mathematical center. That way we can consider the right angle triangle formed by the eye, the center and the object. That allows us to use simple trigonometry.

Now, so we are all on the same page here, I shall refer to the parallels as being 0º (zero degrees) apart, hence a 0º toe in. And I shall talk about nº, the number of degrees each of the eyes (or cameras) converge, as the nº toe in (n-degree toe in).

This angle can be calculated by getting the arcustangent of the distance of the object in millimeters divided into 30 (the distance of the eye from the center), then multiplying the result by 180 and dividing by π (pi = 3.14...). Bear with me, I do not expect you to do the math, I'll do some for you. Of course, if you want to do it, more power to you. :) (and in that case the PowerToy Calculator for Windows XP is your friend).

So, if we are looking at an object that is 30 mm in front of our eyes, we do not even need trigonometry, we know from basic geometry the angle is 45º, something we can achieve very easily with two cameras. But, hey, how often do we look at something that close? Once a year during the eye exam perhaps.

What if we are shooting an object standing 2 meters (2000 mm) in front of the camera? That is a good close-up that can happen a couple of times in a movie. So, we enter atan(30/2000)*180/pi into our trusted PowerToy Calculator, press Enter, and instantly the result of 0.859... appears (the calculator itself uses an insane number of decimal points, so I only copied a few. Yes, folks, that is less than 1º. Can you even use such a precise toe in and converge each camera by 0.859º without using some microprecision gear?

Well, what if the object is 3 meters (3000 mm)? Then the angle is 0.57º. For 5 meters it is 0.34º, For 10 meters, 0.17º. For 20 meters, 0.086º. That is less than a tenth of a degree, so I'll stop here.

I think by now you get the point. As far as our eyes are concerned, infinity is so close that any toe in, if done right, would require extreme precision. Can you even tell if your two cameras are parallel within an error of less than 1º without using some very expensive and very precise angle measuring equipment?

So, now you understand why I have always said to stay away from toe in and keep using the inter-axial distance adjustment for whatever 3D needs you have.

Maybe someday someone will start manufacturing precision equipment that allows us to use toe in the same way our eyes use it. When that happens, I will reconsider my advice (though what I have described is not my only objection against it). But for now, my answer to the question, especially considering it is usually asked by people new to 3D, is steer clear from toe in.

And of course, all rules can be broken. But you should only break them given two conditions: (1) You understand why the rule is in place, and (2) You are breaking the rule to achieve a special effect. So, as long as you know exactly what you are doing and what you are trying to accomplish, by all means do it. But for everything else, toe in is a bad idea, or at least an idea which is not yet technically feasible to do properly.

Thank you for this nicely argumented post which was easy enough to understand even for a doofus like myself. It was also nice because my 3DFF Indie rig has no convergence adjustment at all - in fact, it doesn't even make sure the cameras are both pointing straight ahead at zero toe-in or toe-out. A slight problem there, as you can imagine.

So, using I-A exclusively. What was the formula again? Distance to picture plane divided by 30 or so equals distance from lens center to lens center?

The thing that gets forgotten again and again is that we are not creating 3D images but a pair of 2D images presented in 2D to each eye. We are creating an optical illusion, not a 3 dimensional image. Because of this it is not always desirable to exactly mimic they way our eyes work or what they would see in the real world. In addition what seems to have been forgotten in Adams explanation is what is happing to the image our eyes see beyond the point of convergence. Even if the angle of toe in is minute when we converge on an object that is 10m away, objects that are 20m away will have the equivalent of 65mm of disparity. It's not all just about what is converged but also what is diverged, beyond the convergence point. If I was talking to someone 2m from me and there were mountains several miles distant there will be the equivalent to a large amount of disparity in those mountains in my visual system, whether I am conscious of it or not. There will also be differences in the parallax seen by both eyes with each eye seeing a slightly different vanishing point. Parallel shooting does a very poor job of mimicking this.

While I can't easily measure the angle that my cameras are set to on my rigs to sub 1 degree increments, I can measure the foreground and background disparity on my monitor and this is what is important. I can then make the tiny fraction of a degree angulation adjustments and I-A adjustments necessary to give me full control of the depth of my scene, convergence and roundness while staying within the appropriate disparity limits for my target audience. Using a mix of I-O and angulation allows me to adjust the roundness of the image which changes with focal length and to compensate for image foreshortening at longer focal lengths. If you only shoot parallel it's far harder to control roundness, convergence and disparity together. The narrow I-A required for convergence in close up shots if you shoot parallel often leads to a very flat image. Simply increasing the I-A a little and adding a small amount of toe in will make the image rounder and deeper. This undesireable effect is particularly noticeable on faces shot close up with parallel cameras where a narrow I-A has had to be used to allow for close convergence.

I don't shoot stereo video (yet?) so don't have an opinion about when or if one should toe the cameras in a given situation.

However, stereopsis is effective to around 100m in a person with a normal visual system...no tropias and 20/20 "best corrected" vision in both eyes. So at least for a matched pair of eyes, "infinity" is generally 100m or more, not 1m even though the calculated arc that is subtended is quite small. Apparently some parts of our brains are pretty good at geometry, no matter our classroom grades!

Your 3DFF rig will do toe-in, here's how. Set both cameras at 0 - have them lined up so there is no overlap to them and you are seeing essentially a 2D image on the 3D monitor in superimpose mode. Move your upper camera to the desired I-A distance and now undo the two round 3DFF camera plate knobs on the front and back of the plate of the Lower Camera and rotate it ever so gently to the left until the disparity of your foreground image is resolved and it looks more like a 2D image at that point (usually the point of foreground focus) and you have now converged on that point.

If you don't have these plates, buy them from them (overpriced I know) ; http://3dfilmfactory.com/index.php?o...d=56&Itemid=79 top item on this page - then you can converge with caution.

Alister, not to hijack this thread, but will your Hurricane rig have measuring indicators for I-A and convergence? Are you still on target for sales this year?

Alister, I cannot think of anyone more fitting what I said about breaking the rules. You are a very experienced 3D filmmaker, you probably know what you are doing better than any of us here.

I aimed my message at the people who come to the forum trying to learn the most on how to get into 3D. They should stay away from toe in as a plague. They need to learn to work with the interaxial distance before they do anything else. Once they have as much 3D experience as you, they will be able to tweak all the variables. Until then, toe in will only cause headaches to both themselves and their audience.

Nice discussion, everyone.

while some number look small (the angle in degree calculated here), you have to take in account they can influence other numbers by several magnitude. (especially the TAN function)
So a 0.5 degree of difference can represent A LOT of horizontal shift if far enough.
this leads to windows violation (object on one view are not visible on the other view) or impossibility to overlap distant objects (ghosting)
The easy way is to shoot with parallel views and very small I-O distance, something that seems a common acceptance today.

Hey Adam. There is nothing wrong with shooting parallel as such and it is certainly one of the safest ways to shoot S3D, especially if you don't have accurate monitoring. Indeed one of my handheld rig is fixed I-A (35mm) parallel for run and gun type shooting.

I favour a hybrid shooting method that uses (in most cases) a fixed toe in for the scene and then uses I-A for convergence. I start by setting up for the deepest shot in the scene. With the cameras set to zero I-A. I then toe in the cameras until the measured on screen background disparity hits the limits I am shooting to. So at this point I have convergence on the sensor plane and the background diverged by my on screen maximum (hypothetically 65mm on screen). This means that no matter what else I do, provided I don't change the angulation (convergence) I will never exceed by diverged disparity maximum.

Then I bring the cameras apart and use the I-A to set the desired convergence point. The further apart the cameras, the further away the convergence point will be. One of the beauties of this method is that it is pretty "safe" and you don't need to constantly adjust the angulation. It also works well when cutting between close converged and distant converged shots as the overall scene depth only changes by a very small amount compared to what would happen if you used the toe-in (angulation) to adjust the convergence. Angulation changes have the effect of compressing and stretching the scene and cutting between different scene depths is un pleasant for the viewer.

It's not always as simple as this as there will be shots where you need to compensate for roundness or to force a little more or less depth. But for most shots this works very well and is straight forward to do, provided you have some way of monitoring your S3D output. This could be as simple as a regular monitor with an A/B switch to flick from one camera to the other and a ruler or grid on the screen (chinagraph pencil of printed on acetate?) that allows you to physically measure the disparity. If you work in percentages it's very easy to scale up and down for different screen sizes. I use a Transvideo Cinemonitor 3D which can add grids etc so it's easy, but it's not a cheap piece of kit. I like the idea of using stereoscopic multiplexer on a tablet PC with a couple of USB dongles, that would work too.

My Hurricane Rigs will have calibrated I-A scales. We are working on calibrated angulation, but there is huge variation in the lens centering of most video cameras and it will depend on how accurately you mount the cameras to the quick release plates. In fact as you change the focal lengths of most lenses the centering drifts from side to side as well. We hope to have a pointer and adjustable scale so you can at least see whether you have more or less toe in from shot to shot. Angulation will be done by a single knob on the rigs. Really though there is no substitute for a good monitor. As with all these things we have had a couple of delays. I should be in possession of the first 4 pre-production rigs in the next two to three weeks. Once these have been thoroughly tested we will have a small production run that will again need to be tested before full scale mass production. I'm still hoping that we will be in full swing by late September and we are still on target for a truly stunning price. We've even managed to add a couple of very clever tricks that no other rig I know of can do, but I can't tell you what yet as it's subject to a patent application.
We should also be releasing the dual focus kit and motorisation kit for the rig at around the same time. The motorisation kit will add full remote powered I-A and angulation adjustment, but for most applications this isn't essential.

Thanks for posting, Bruce. OK, this is off-topic for this thread for which I apologize. Bruce, my Indie rig does not have those plates, and yes, they are overpriced. So was the Indie rig, horribly so. Unfortunately all this shooting 2D/3D video lark is only a hobby to me and I need to finance it from my other income - which does not exist at the moment, as I was laid off back in February. So, yeah, not going to buy those plates :)

Petri, you only need one of them for the bottom camera and although they are overpriced, I think you can call Karl over there and see if he can do better than list $ for you. If you get one, your rig will be 2X more efficient at getting quality 3D than without them. You've gone this far . . .

Well that's a really interesting post Alister.

OK please bear with me if I totally misunderstood you:

Do I understand correctly that withe cameras at 0 I-A you set a toe-in for the most distant part of the shot which diverges everything to the max parrallax you can accept for your set-up & final screen size etc. This will diverge the background. Then you pull the cameras apart until they converge on your chosen convergence point? This puts your background somewhere between pure parallel and converged and this method also determines the I-A instead of say consulting a table or using a 1/30 type rule. Would this also mean your background will have a bit less than maximum allowable parallax and just a touch of keystoning?

If i have this right, then I was wondering that you said you set this for a "scene", not just a "shot". Assuming you are maybe using different focal lengths and moving the camera around in a scene would you keep the that intitial tow-in the same?

I gather also that maybe you adjust for shots that are more extreme - long or short lenses or close or more distant shots?

Quoting your previous post by "close-up" do you mean that as in the close-up of an actor or interview subject in a scene- i.e typically a somewhat longer lens shot, or did you mean a shot that is just close to the camera? Sounds like the former:

"The narrow I-A required for convergence in close up shots if you shoot parallel often leads to a very flat image. Simply increasing the I-A a little and adding a small amount of toe in will make the image rounder and deeper."



BTW - I'm not going to try this yet, I'll remain safe with parallel for a while, but I will test it.

Thanks

Lenny

Hang on, bottom camera? Oh, I see. No, I have the side-by-side 3DFF Indie rig, the cheapest of the lot at $499. Couldn't afford anything fancier.

Hi,

I work with 2 HV10, minimum base distance 6cm.
When recording close subjects, i have to toe in, because when parallel,
at editing i have to crop from left and right of both pictures.
Then if i want the full 1080 resolution back, i have to zoom in, and that degrade quality.
Instead i converge the cameras, and for me it is not hard to view.
When there is big disparity in the far away objects, use smaller DOF.
Like in Avatar :)

"This could be as simple as a regular monitor with an A/B switch to flick from one camera to the other"

That's exactly what i want to make last week!
With the very simple circuit, that switch from two composite input to one output, the display has to be in sync, and that's a half sec noise in picture when switching, so i cannot compare the two video.
I think i need a genlocked circuit. How hard is to make such a circuit?
Marton

Leonard: Yes you do seem to have understood me correctly. My starting point at zero I-A does give you a scene with everything diverged, but that is only for setup. My background disparity will always be less than my maximum allowable, that's the beauty of this method, it's difficult to mess up! Convergence is adjusted using a monitor and not tables, although generally you find that the I-A etc will tie in pretty closely with most converged calculations.

Because I'm toe'd in there will be some small keystone errors but these are normally tiny as I'm generally working to 2% average - 4% max disparity limits which results in quite narrow I-A's.

Yes by close up I did me a shot where a longer focal length is being used to shoot a head and shoulders type shot, but the same flattening can also happen if you have a narrow I-A (less than 60mm) and the cameras are very close to the subject.

By scene I mean sequence of shots that will be cut together and represent the same location or subject. Change subject or location and you need to re-set. Also changing focal length will require a reset, just as you would if you were using a calculator or tables. One of the challenges is getting from say an interior scene with only 20ft of depth to an exterior with many miles of depth. This is where other depth cues really help such as scale. So for my last shot in the room I would try to include a person in the front of the scene (not in positive space, just well away from back of the room) and then in the following exterior have a tree or something else in the foreground. These scale cues then tell the brain how to interpret the stereoscopic cues it's getting. When working in a narrow disparity range you tend to use almost all of it for every scene, so if you only have stereoscopic cues the interior shot would appear to have the same depth as the exterior landscape. But fortunately scale cues get used by the brain for depth more than just stereoscopic cues. The stereoscopic cues tell our brain what's in front or behind, or that "A" is twice as far away as "B", but scale is used by the brain to work out what the distance between A and B is.

Prech: Good old fashioned CRT Tube monitors without any fancy electronics work best for A/B switching as they switch almost instantly and will re-sync without any big delay.

Thanks Alister,
Very helpful information and it sounds like a very logical way to work.

It's been a treat to have the benefit of all your experience both on this board and on the EX-1 board where I've been following your posts for a few years. I have a few more related questions if you don't mind, but I'll take a little while to formulate them.

Lenny

This is a great thread. Let's say if one is shooting a simple drama with no visual effects or explosions or gimmicks, will I get acceptable theatrical results by just using an inter-axial rig and no toe in at all?

Mostly shot on wide angle in rooms without moving walls but not handheld. What should my main focus be in this case? Would I be better off with wide angle or long lenses in this case?

Even easier but less accurate - you can loosen the camera mounting screws on one of the plates you have and toe one camera in towards the other until the foreground images (if that is where you are focused) seem to merge together and get reasonably sharp. That is basic convergence right there.

You should get perfectly acceptable shots either parallel or toe'd in. Provided the camera are truly parallel then that's the safer way to shoot if you are at all unsure about your abilities or rig setup. Toe in gives you more control over on screen depth and usually give a slightly rounder image which looks more natural, but needs more care and good on set monitoring.

The longer the focal length of the lens the more obvious any rig alignment errors will be. For most S3D shoots I will try to keep to a focal length of less than 75mm (on a 35mm sensor). Typically lenses that come close to the human field of view make shooting good S3D easier. Very wide lenses will be problematic with mirror rigs as you will often end up getting the extremes of the mirror or second cameras in the shot and with side by side rigs it can hard to get the cameras close enough together. A very wide lens will give you very flat looking 3D, so you end up moving the cameras apart to add depth, which then leads to cardboarding. 35 to 50mm is an easy range to work with.

I'm helping to run a 3D event in Chennai at the end of the month.

Yikes, I thought it was the opposite , that a long lens led to flat looking 3D and cardboarding.

Yup, I know... but I have nothing but the cameras and the rig. No external monitors, zilch, nada, zip.

Cardboarding or the cut out effect is one much discussed and debated. It occurs whenever the relationship in a shot between the total image depth(volume) and relative depth of individual parts of the scene are miss matched. It often occurs whenever there is a conflict within the scene between the scale of objects and their relative stereoscopic distances. It can occur at any focal length. It is very common with long focal lengths, as compared to what we normally see with our eyes the image is foreshortened or compressed. This has the effect of squashing the image into a series of flat looking planes. To compensate for this it is traditional to increase the interaxial as this increases parallax which then lengthens the squashed planes. With medium length lenses this can work, but it will depend on how far the subject is from the camera. A subject close to the camera will gain more roundness as the cameras sees more of the modelling of the individual object but subjects a long way from the camera will not become any rounder. Moving the cameras apart will compensate for the lens foreshortening as this increases the depth or volume of the scene, but if the planes within the image have no roundness because they are too far from the camera then all that happens (IMHO) is that the increase in depth or volume simply separates the flat planes still further creating a very un-natural effect. In addition exaggerated interaxial's lead to miniaturisation where the brain is confused by the scale of objects compared to their apparent distances. With very long focal lengths cardboarding or the cut out effect is very hard to eliminate. This planar effect is dependant on many factors, focal length, interaxial, scene depth and how near or far subjects within the scene are to the camera.

Going back to very wide lenses the opposite can occur as these will tend to give a scene with a small on screen volume relative to the depth of the real world scene. If you have a nice range of subjects in your scene, some close to the camera, some further away then generating a good stereoscopic image will be straight forward. However if everything in your scene is on a similar plane or close to the rear of the scene then the image may look very flat or lack volume. So to compensate the temptation is to increase the depth (volume) by increasing the interaxial. This "stretches" the scene depth or volume, exaggeration the separation between objects. If the camera is not close enough to those objects to show increasing roundness then cardboarding or planar effects will once again become apparent.

Of course this will happen at any focal length. So you must balance focal length and interaxial with both scene depth and subject distance. A wide angle lens will work best when you are close to your subject, but is not a substitute for a narrow interaxial. I like to try to work at FL's between 35 and 75mm (35mm equiv) and would much rather move the camera closer to the subject or further away than go outside that range. Obviously there are many time when you do have to go longer or wider and this is when creating a good stereoscopic image becomes much more challenging and much more subjective as in either case we are no longer presenting an image anything like the images seen by us humans.

Thanks again Alister for the best explanation of "cardboarding" I've seen anywhere else. This is helpful to me. a great thread. Thank again to everyone.
By the way, I just played with the method you suggested earlier ( for setting convergence with a slight toe in based on the furthest distance ) for a few minutes yesterday while mainly working on rig issues and it sure seemed to me like an elegant way to work on the set and see what your getting right away on the monitor without the other disadvantages of full convergence.. With parallel we have to go in to a Davio Box and readjust the horizontal offset constantly to get the convergence right for the monitor. Not that bad but slows us down with a small crew.

You are correct.
The wide angle setup will have normal depth in the center but progressively going to the left or right edge the volume gets contracted or squashed to 0 at 90 deg due to effective interocular distance decrease.

Mathew Orman

Wrong assumption.
Wide angle shoots require wide angle viewing or else gross perspective distortions defeat the purpose.

Mathew Orman

"cardboarding" is due to viewing narow view angle shoots with normal view angle. Example: 10 deg telephoto and 40 deg on monitor.


Mathew Orman

Cardboard predominantly occurs when there is an imbalance within the on screen image between the total image depth or volume and the apparent distance between the objects in the scene.

Viewing an S3D screen using different FOV's will not change this relationship as what we are viewing is a pair of flat images, not a true three dimensional scene. All it will do is make the image bigger or smaller which will increase/decrease the apparent disaprity, but the relation ship between the dissparity and on screen distances will remain exactly the same as what you are looking at is essentially a 2D image not a 3 dimensional one.

I don't follow you at all Mathew. If you shot a distant object with long lenses against an even more distant background to give some depth to the scene you would most likely use a wide interaxial. This shot would almost certainly have some cardboarding. Lets consider a shot of a soccer game from the back of the stadium. The camera is 150ft from the pitch using a long lens and lets say 18" interaxial. In the shot you can see the width of the pitch and a couple of payers. The two players A and B appear to be of similar size (due to foreshortening) even thought they are on opposite sides of the pitch. The players themselves show little depth as the cameras are simply too far away compared to the interaxial to resolve any significant depth in the players themselves. Our parallax depth cues however are telling us that the players are 40ft apart as the disparity difference between the front of the pitch and rear of the pitch are quite large due to the hyperstereo. Our brain will interpret this as flat cardboard cutout players on a pitch with some depth to it.

Mathew, are you suggesting that if I went to the cinema to watch this using a pair of binoculars this would some how help? The magnification introduced by the longer focal length of the bino's would increase the apparent disparity thus increasing the volume(depth) of the pitch, but the players will still appear flat as the cameras were not far enough apart to resolve any meaningful 3D in them, the relationship of the size of the players will stay the same and the parallax will also increase so if anything it will make the situation worse not better.

If we were projecting a true three dimensional image then I would agree with you, but we are not. We are showing flat 2D images presented in such a way as to create the illusion of depth and changing the scale of these flat 2D images or magnification of the them will not alter the ratio between parallax, scale or disparity within those 2D images, that was determined when the images were created.

"Mathew, are you suggesting that if I went to the cinema to watch this using a pair of binoculars this would some how help? "

No it is the opposite, you would match the camera's view angle by using -5 diopter
correction glasses. Which would give you a screen size appropriate to match the 10 deg telephoto image.
Human eyes have no ability to zoom.
Telephoto stereo shots are only good to emulate using a pair of binoculars
and in such case carboarding is natural as experienced by many people.
Appropriate use at cinemas should include circular mask to inform viewers that camera is emulating binoculars.

Also if you want to see cardboarding of flattening by mismatched view angle then just get you face about 3 inch from your desktop monitor.
Opposite happens (excessive stretching) if you get away from normal position something like 10 feet .

Mathew Orman

So your proposal Mathew is that we only ever shoot and display 3D using ortho-stereo or provide the viewer with corrective glasses to match the FOV of the camera?

Corrective lenses will still not eliminate cardboarding as I described and as being discussed as the cardboarding is recorded in a 2D image, so no corrective viewing devices will eliminate it because the ratios contained in the image are incorrect. Magnification or reduction of the image will not change those 2D ratios, only the scale. This will reduce or expand the distance between the flat planes, but they will still be flat planes because unless you had an extremely wide interaxial (with associated disparity issues) in my soccer game example the camera separation would not have been wide enough to resolve any depth in the players themselves. We are talking about the real world here with limited camera and lens resolutions (and thus depth resolution) and not a hypothetical world with unlimited resolution.

"Corrective lenses will still not eliminate cardboarding"

Wrong again, corrective lenses make distant object blend with background
totally flat as perceived by human eyes. So there is no cardboarding effect observed.
Corrective lenses with negative diopter are used in the cinema system that has the sweet spot seat at the last row and in such system every one has the same FOV that matches displayed content.
Also, orthographic stereo images are always made with orthographic cameras.
Matching stereoscopic geometry between viewer and the content has nothing to do with orthographic stereoscopy.
I suspect that you have never seen an orthographic stereoview.
But if you did then please provide an example.
Finally you can test your stereoscopic geometry knowledge
by providing an answer for a simple question:

What is the infinity parallax in inches for a screen that is 30 feet away?

Mathew Orman

Thank you Alister...that was the answer I was looking for. Can you tell me more about the Chennai event? Would love to know more. Thanks.

In 3D movie making by Bernard Mendiburu as I recall he suggests never using a lens longer than 30mm, but he doesn't say what format that refers to. I took that to mean its the lens angle that matters not the format.

This implies you'd get a wider range of lens choices on a smaller format. i.e 2/3" video would be easier to shoot 3D than 35mm and a 1/2" chip or 1/3" chip even easier.

His advice sounds way more conservative than what Alister is recommending ( 35 - 50) but that book is woefully short on addressing these kinds of issues and maybe he did have a format in mind. Way better info here.

lenny levy

Well yes of course, if you make the image small enough that you can no longer resolve the disparity in any of the image then yes it will be flat, without cardboarding, but it will also be tiny. But one thing your forgetting is that finite camera resolution limits depth resolution. As the cameras would have been unable to resolve the depth in the players themselves so they would still be flat planes until eventually you corrected the image enough to make the entire image flat. It's arguable that cardboarding can occur in 2D images as well as 3D images. Long shots done with tracking cameras or camera jibs can appear to break down into planes as the camera moves because or brain is confused by the depth cues it is seeing.

This discussion is about how S3D is currently made and viewed, it's not about hypothetical theories about perfect stereoscopic vision replication, which seems to ignor the way our brain works using combinations of depth cues, not just matching focal lengths and viewing distances.

Assuming the viewer is perpendicular to the screen and has eyes 65mm apart, then 65mm I don't need to calculate that.

Mathew,

Maybe I'm dense but i don't understand what orthographic stereo is, and it sounds like your describing a type of cinema that doesn't exist in most of our practical experience. Does what your talking about have practical application?

Also I hope we can keep this discussion respectful and courteous. Its been the best discussion of the subject I've seen anywhere so far and I'd hate for it t get sidetracked.

Leonard. Sensor size makes no difference it is the field of view that matters because when you deviate greatly from a natural FoV it's much harder to produce realistic looking images.

At the moment no one is really sure exactly what is acceptable or desirable. Do viewers want a natural image or do they want forced depth. For a narrative piece you possibly can stay very natural, but sports in particular present all kinds of problems some of which come about because of the limited resolution of the cameras and viewing systems. I would not want to watch a soccer match all shot with wide shots on TV. Longer shots do take you closer to the action but then you have cardboarding and other undesireable effects. In 2D we just accept foreshortening as normal but in 3D it is more objectionable.... or is it? Do we accept it and live with it because forced depth helps tell you where the ball is going in 3D space. These are questions that are still un answered.

Othographic reproduction is the 1 to 1 reproduction of something. So ortho stereo would have cameras that have human FoV toed in on the most distant subject in the screen. The footage is then shown on a screen that matches the human FoV with everything in negative parallax so what you see is an entirely natural view.

The problem is that for true ortho stereo it requires that the screen is as far away from the viewer as the most distant object in the scene as everything is in negative parallax and thus in front of the screen. That's fine for a fixed scene of say a room, where you could match the viewer to screen distance to the back wall of the room. But when you are looking at exteriors with massive depth that presents a problem as the screen would need to be miles away and massive. So in practice you place infinity (or your back wall) behind the screen so now you have both negative and positive parallax but now it's no longer true otho stereo as to do this requires some kind of toe in via the projectors or electronically which can then introduce some of the issues we are all familiar with. True othos stereo also only has one single viewing position, the one that matches the camera so only the one person sat in that spot will see an ortho stereo image, everyone else see's something not quite ortho stereo.

The nearest I've seen to ortho stereo is some of the early IMAX 3D films where they tried to match camera FoV to Human FoV. In the early days this was pretty close as all IMAX screens were the same size and the stadium seating means there is not so much variation from seat to seat in screen to viewer distance. These days it's not so good.

Yes,
and currently S3D is totally distorted for the sake of parallax size on the big screen since if it is OK on big screen it will be miniaturized on all small ones and parallax will be always less than infinity for the backgrounds.

This type of fallacy and disinformation of public is now being rejected and exposed. See example:

3D, Who Needs It? | John C. Dvorak | PCMag.com

Also your calculation shows lack of understanding of real geometry of human stereoscopic vision.
The infinity parallax for screen that is 30 feet away is: 860 mm or 34 inch

Mathew Orman

And exactly how did you come to that figure? How far from the screen is the viewer seated? Our eyes do not diverge when viewing infinity.

Perhaps I don't understand the true geometry of human vision, but I do know what can and can't be done in an S3D production and what does or does not look good to the vast majority of people and this is what is important. Film making is a trick, it is a gimmick. We use actors because thay are good at exagerating certain phrases or actions to help enhance the story. Take explosions in films, they are normally full of fire and flame when a real bomb tends to be a shock wave with little fire. But which does the viewer enjoy watching, big, dramatic fireballs or shockwaves?

Are you seriously telling me that I and just about every other person making S3D films is wrong and that you are the only person that is correct. You do seem to forget that we are not creating 3D images but pairs of 2D images and creating an optical illusion that does not just rely on human vision geometry but also uses other depth cues, in particular scale. It's very easy to trick people into thinking they are looking at objects that are infinity simply using scale or even removing any other depth cues. A well set up planetarium for example will have people believing that they are looking at stars at infinite distance even though the projected 2D dot is only 30ft away. Our eyes do not measure distance they estimate depth ratios and relationships our brain then compare how far A is from B and then using scale judges depth. That's why golfers find it hard to judge the distance to a green unless there is a flag to help with scale. Put a different size flag in the hole and see what happens.

We are generating an optical illusion, not a mathematically correct 3 dimensional image and like all illusions there is more to it than pure science.

Yes, Mr Dvroak is right about DOF, but unfortunately DOF is the Hype today with DSLR.
Historically DOF had some reasons to exist, but not today (except for some artistic will).
It is really hard to match DOF and S3D to give a watchable picture. I think Avatar was not so bad at it.
Unfortunately, as soon as you are shooting real world, you do not get the same possibilities than getting pure computer rendered scenes.

Wrong again,
the requirements are that camera geometry matches the viewer geometry and in such stereo window size that is defined by camera geometry is the same as the projection screen and both FOVs are the same.
The toe-in after keystone correction is no different than off-axis camera.
What is in front or behind the screen is only defined by stereo window position within a scene.
Finally both toe-in and off-axis camera geometry do not match real geometry of human stereoscopic vision.
And it looks like none of the stereographers can define the correct stereoscopic camera geometry to match human vision.
That is why when one ask to define infinity parallax they always quote the human eye distance.

No orthographic stereo view example yet?

Mathew Orman