Sam wrote:

>Oleg, now you've really confused me. What exactly to
>you agree to with Chuck and disagree with me?

I have to apologize. I should not have mentioned names. I was hoping this
discussion would shed some light on the physiology and psychology of
viewing stereo.

>So does that mean you feel the camera DOES see the
>world as the eyes see it?

Exactly. Let me restate my thought experiment. Forget about stereo for a
moment and think about what just one eye sees. Suppose you were in the
desert and constructed a large wall with a wall with a six foot square
window cut into it. Now you stand six feet behind the window and look with
one eye at the landscape beyond. Place a camera at the exact position of
your eye and photograph the window. Enlarge the photograph to six feet
square and insert the print in the window frame. If you could illuminate
the print brightly enough, your one eye would see almost exactly the same
landscape were you to remove the print. Now make similar print from the
position of the other eye and insert it in the window frame. I submit that
if you could force each eye to see in the window frame only the print it is
supposed to see, your mind will perceive a scene that is spatially
identical to the landscape behind the prints.

>Then how do you explain the keystone distortion issue,
>which does not exist through your eyes, but is as plain
>as day in a photographic image?

Yes, keystoning does enter into my thought experiment, but only as a
secondary effect. The prints in the window frame will not be exactly
square. Without actually doing the math, I imagine that there might be a
quarter inch difference in the heights of the prints at the left and right
ends of the window. If you want, you could always correct that bit of
keystoning in Photoshop, similar to the way you would correct a phantogram
to appear to lie flat on a table. But at this distance it is hardly worth
the effort.

>> How many times have you shown a stereo view to a complete
>>novice only to have him or her exclaim how real it looks?
>How many "novices" have said an ugly baby was actually ugly?

I can't believe all those people, both novices and jaded stereo buffs, from
whom I have had to wrest away my viewer were just being polite to me.
Actually, I only cited the example to illustrate how easy it is for most
people to decouple their focusing reflex from their eyeball convergence.
There must be some people for whom this feat is difficult, but I suspect
it is a small minority.

>Are you saying the ONLY reason the large majority
>of us cannot view the stereo image Chuck is
>suggesting is because of viewer lenses?

No. I have not seen Chuck's image. Images I have found painful to view
were vertically or rotationally misaligned, or had the chips mounted too
far apart, or had focal length mismatch. I do believe, however, that the
effect of viewer lenses is underappreciated. One of the problems is that
as the eyes rotate to converge on a homolog they also shift a bit off the
centers of the lenses. The closer the subject matter the more they have to
shift and the more degraded the view through lenses becomes. But the eyes
are not stupid. They will not linger on subject matter that is painful to
view. The trick is to is to keep such subject matter from the away from
the center of the view. A mirror based viewer should yield a more
comfortable view than a lens based viewer, at least with respect to
convergence. But with mirrors it is difficult to maintain alignment of the
left and right images.

>Are you saying that if that exact stereoscopic image
>was enlarged to a pair of 4 foot square prints and
> viewed through a mirror stereoscope, we would see
>it as perfect as in real life?

Precisely, as long as it is properly aligned and illuminated.

>Why then does it also not also work in a projected
>stereo pair or an anaglyph?

It does. I have seen giant anaglyphs that look beautiful. As for
projection, I remember the IMAX space station movie. It opened with a
simulation of an astronaut stepping out of the station from his own point
of view. You could see his hands grasping the ladder right where your
hands would be. I extended my hands to meet the image of the astronauts
hands. The image of the astronaut's hands was at exactly arm's length. I
suspect that most of the audience looked past the hands - the view of the
earth beyond was so compelling - but that they could have converged on the
hands had they thought of it. For me it was easy. But I do recognize that
what is easy for me might be difficult for others. The trick, as with any
art, is to make sure that the reward is worth the difficulty. Ideally, a
work has several levels of difficulty, so that it appeals to audiences of
various levels of sophistication.

Oleg Vorobyoff

Oleg,

I think you have missed one vital part of this discussion. The issue
is not whether the camera and eye are similar in a STANDARD depth
image, but one that has detail 18 inches to infinity away!

Your argument below is for an object of significant distance from the
viewer. You have mentioned a window 6 feet away, but did not meantion
that there will be an object 18 inches away jutting THROUGH the window!

This is what I disgree about. I think we all agree that the infinity
spacing should not be beyond 65mm (for me even 63.5 is my preference)
to eliminate divergence and the discomfort it brings almost everyone.
But if there is detail 18 images away, the OFD must be huge (sorry,
couldn't figure out how to use the Excel, but it must be 6 or 7mm perhaps)

The proposed solution (OK, no names) as I understand it, is "double
depth" which is basically extending the foreground and window to
accommodate. To me this isn't the solution. To me the solution is
simply not to take images with this ridiculous amount of depth with a
fixed base stereo camera. Double depth will certainly work to a
degree, but not to this extreme.

This has nothing to do with the method of viewing, whether optically,
by mirrors, etc. It's the basic rules of excessive deviation. Does
this make more sense?

Sam

--- In MF3D-group@yahoogroups.com, Oleg Vorobyoff wrote:
>
> Sam wrote:
> >Oleg, now you've really confused me. What exactly to
> >you agree to with Chuck and disagree with me?
>
> I have to apologize. I should not have mentioned names. I was
hoping this
> discussion would shed some light on the physiology and psychology of
> viewing stereo.
>
> >So does that mean you feel the camera DOES see the
> >world as the eyes see it?
>
> Exactly. Let me restate my thought experiment. Forget about stereo
for a
> moment and think about what just one eye sees. Suppose you were in the
> desert and constructed a large wall with a wall with a six foot square
> window cut into it. Now you stand six feet behind the window and
look with
> one eye at the landscape beyond. Place a camera at the exact
position of
> your eye and photograph the window. Enlarge the photograph to six feet
> square and insert the print in the window frame. If you could
illuminate
> the print brightly enough, your one eye would see almost exactly the
same
> landscape were you to remove the print. Now make similar print from
the
> position of the other eye and insert it in the window frame. I
submit that
> if you could force each eye to see in the window frame only the
print it is
> supposed to see, your mind will perceive a scene that is spatially
> identical to the landscape behind the prints.

Sam wrote:

>I think you have missed one vital part of this discussion.
>The issue is not whether the camera and eye are similar
>in a STANDARD depth image, but one that has detail
>18 inches to infinity away

I thought that the 18 inches was cited only to prove that the unencumbered
human eyes can easily converge on subject matter that close. I did not
realize that there was actually a slide that exhibited that much depth. I
take "double depth" to be precisely that: for 35mm it means an on-film
deviation of twice 1.2mm, or 2.4mm, and for MF twice 2.5mm, or 5.0mm. I
believe deviations of that scale can be enjoyed by most people if
sensitively presented.

I do still think that an ordinary stereo camera can produce a stereo pair
acceptable to the eyes with subject matter as close as 18 inches. It seems
obvious if I modify my thought experiment. With a piece of clear mylar
mounted on a window 18 inches away and a lot of patience I should be able
to trace out exactly what either eye sees behind the window. But a camera
could do that task better than I can. However, in the raw prints there
would now be noticable keystoning of the window frame. A corrected set of
prints would need to be produced with the outlines of the window made
square. With these prints and a non-distorting viewing method, such as one
using mirrors, the eyes should be happy, since every shape they receive
from the virtual scene is identical to each shape in the actual scene. I
believe that practically all pain in viewing stereo is caused by shape
distortion or misplacement. The brain is exquisitely tuned to interpret
subtle differences in shape as measurements of depth. Give it an unnatural
pair of shapes or shift one unaccountably, and it goes nuts. A fine point
that still bothers me is that eye spacing decreases a couple of millimeters
as the eyes converge. So it would be impossible to make a single set of
prints that would present both nearby and distant subject matter to the
eyes exactly like they expect it. At 18 inches to the subject, however, it
does not seem that camera lens positions off by only a millimeter would be
enough to confuse the brain.

>This has nothing to do with the method of viewing,
>whether optically, by mirrors, etc. It's the basic rules
>of excessive deviation.

I guess I still don't understand the fundamental basis of those rules. If
there is nothing fundamental standing in the way, there should be ways of
breaking the rules, and to produce better pictures along the way.

Oleg

I had written:

>I do still think that an ordinary stereo camera can
>produce a stereo pair acceptable to the eyes with
>subject matter as close as 18 inches.

Having thought about viewing magic eye stereograms, I have to modify the
above statement. I still believe that the camera can capture perspectives
precisely like the eyes expect to see them, but there remains the question
of linkage between the convergence of the eyes and where they are focused.
It was agreed on this forum there is no hard linkage - viewing stereo
pairs would be impossible if that were true. However, my recollection is
that stereograms demanding a small disparity between focus and convergence
were easier to view than those that required the eyes to diverge out to
parallel lines of sight. So the linkage must follow something like Hooke's
Law for springs: the more the eyes have to diverge from what their
convergence is at reading distance the harder they have to work.

The basic difference between ordinary sight and viewing stereo pairs is
that stereo pairs need to be read. That is, the eyes initially need to
converge onto the virtual or actual plane of the stereo pair, like they
would converge onto the text on the page of a book. Having accomplished
that perfectly natural task they need to do something quite unnatural -
diverge a bit, while continuing to maintain focus at reading distance.

My recollection of viewing stereograms (I have no access to one at the
moment) is that there was no pain - not even real discomfort - just hard
work. And doing hard work is often more satisfying than doing nothing at
all. So my question becomes: to what degree is the aversion to viewing
extreme stereo pairs simply due to lazy eyeballs, or whether some actual
physical pain or eye damage is possible. Speaking for myself, I find
stretching out my eye muscles quite satisfying - something like
participating in sports - but I would not want to demand that others do
likewise if it might hurt them.

Oleg Vorobyoff

I don't expect anyone is reading these ruminations, but let me register
another thought. I suspect that the brain has at least two distinct modes
of processing what the eyes see. I'll call them the orientation mode and
the scrutinization mode. The orientation mode serves to give the body a
sense of its place in its current environment. The scrutinization mode
allows one to study the details of some particular object in that
environment.

What has that got to do with stereo? Well, I believe these modes of
perception are linked to how much the eyes are converged. Their natural
state in the orientation mode is parallel viewing with focus on relatively
distant things. Their natural state in the scrutinization mode is
convergence at reading distance with focus at some 10 to 36 inches away.
Since a stereo pair needs to be viewed with the eyes focused on a single
distance, whatever the original distance of the imagery, the stereographer
needs to choose under which mode to present his or her work. I submit that
stereo pairs that are viewed on large screens are best presented to appeal
to the orientation mode, whereas those that are viewed in a hand viewer
should appeal to the scrutinization mode.

I do not know how to measure the amount of convergence of my eyeballs, but
I sense that while looking through a hand viewer they are converged at
reading distance. This would give you an advantage nature does not
provide. You can now scrutinze distant imagery with complete satisfaction.
In real life when you see something in the distance that warrants further
study, you will not feel fulfilled until you close in and get a really good
look at it. Looking through a hand viewer you sense that you are already
as close the scene as you can ever get and accept it as presented. This is
not true of a a projected view. How many times have you felt the need to
close in on the screen to get a better look at something? If you did close
in, most likely you saw a blurry image more difficult to interpret than
before. This means the pictures intended for projection need to be
composed somewhat differently than those intended for hand viewing.
Details are the meat of hand viewing whereas they can undermine the
effectiveness of a projected view.

Oleg Vorobyoff

Oleg wrote:

> I don't expect anyone is reading these ruminations, but let me register

another thought. I suspect that the brain has at least two distinct modes
of processing what the eyes see. I'll call them the orientation mode and
the scrutinization mode. The orientation mode serves to give the body a
sense of its place in its current environment. The scrutinization mode
allows one to study the details of some particular object in that
environment.

>What has that got to do with stereo? Well, I believe these modes of

perception are linked to how much the eyes are converged.

>Snip..... Oleg Vorobyoff

I see a lot of deep thought in your message and agree with most of it. Thank you.

Allow me to give another view which is not far different from yours:

My theory is that the major factor that makes stereo photography work is convergence or parallax. Most of the other depth cues are present in 2-D photos, convergence is no doubt the most meaningful depth cue added with stereo photography. As such, attention should be payed to mounting so as to have convergence in the viewed slide match the convergence the scene had when viewed directly with only our own 2 eyes. Matching the convergence is accomplished by matching the taking lens FL with the Viewer lens FL, the base to eye separation, and setting infinity parallax to zero (Parallel view to infinity). Your mind can then use the parallax to determine distance to objects and make them look to be the real size they were. I see having all the depth in focus as a plus. You can always adjust the focus on the viewer to where your eyes want to be focused (Well maybe not with the 3-D world viewer) at the time. If you do not have a loose coupling between convergence and focus, You can adjust the viewer to the focus your eyes demand while viewing the different depths of the slide.

I see the major problem with projection as being a loss of resolution which makes slides with a lot of detail poor viewing. There seems to be no advantage of MF over 35mm when projecting because the projecting system limits both to the same down graded resolution which is poorer than the originals.

I think that a major problem with viewer viewing and projection is the practice of "Mounting to the Window". It results in a parallax that is not matched to the view as seen directly and varies from slide to slide depending on the distance to the nearest object. It therefor gives the brain wrong depth information while it is trying to converge and resolve the images.

I feel that it was a mistake for 3-D world to try to capture the whole 3-D market by making their system conflict with established norms:

By changing the width of the mounts they make their slides not fit some very good viewers already out there. Their plastic mounts would have been very welcome had they fit existing viewers.

They want to sell their own viewer so they add .8 CM to the slide width. They do not have a focus adjustment to the viewer so it becomes useless to many of us. The viewer FL does not match their camera FL. Surely that could have been done. No back light, a pain viewing indoors.

They spend a lot of money putting lenses on their camera that are sharp at F/8 when it would have been to everyones advantage if they had light weight lenses that matched FL with the viewer, had a f stop of F/32 or tighter for extended DoF, and was optimized for F stops near F/22 where it will be used the most.

Then we are also seeing complaints from owners about unreliability and poor design leading to user modifications to correct problems. Sounds a lot like an expensive Sputnik. I just hope this "China Bomb" doesn't blow up and take all of MF with it.

My $.02US worth,

Chuck Holzner

Maybe it is good that few will read this. %^)







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Chuck Holzner wrote:

> By changing the width of the mounts they
> [3D-World] make their slides not fit some
> very good viewers already out there.

I don't think they knew any "standards" existed
when they started out.

> They spend a lot of money putting lenses on
> their camera that are sharp at F/8 when it
> would have been to everyones advantage if they
> had light weight lenses that matched FL with
> the viewer, had a f stop of F/32 or tighter
> for extended DoF, and was optimized for F stops
> near F/22 where it will be used the most.

a'yup But they are constrained by what MF lenses
are currently available at a reasonable cost.
Yes, my TL120 only goes down to f/22. I'd like
to see f/32. I'd like to carry lighter weight
f/4.5 or f/5.6 lenses around. But, I'd much
rather have all my images look like they do when
they come out of my TL120 than the way they do
when they come out of my Rolleidoscop :)

Have you had the opportunity to mount and view
some images from a TL120, Chuck?
________________________________________
John Thurston
Juneau, Alaska
http://stereo.thurstons.us

John Thurston wrote:

>Have you had the opportunity to mount and view
>some images from a TL120, Chuck?

No I haven't. I did see 2 China slides, neither with much depth. Likely there will be some in folios I will get someday. I have seen some posted on the web but they were shot at around F/8 and so have little depth. Also the resolution they are posted at no doubt does not do them justice.

Does anyone have TL120 images shot at F/22? How are they compared to images out of older stereo cameras? It seems like most people who own them are shooting only the TL120 and so have no side by side comparisons.

We will see,

Chuck





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--- In MF3D-group@yahoogroups.com, "Chuck Holzner" wrote:

>
> Does anyone have TL120 images shot at F/22? How are they compared

to images out of older stereo cameras? It seems like most people
who own them are shooting only the TL120 and so have no side by side
comparisons.

I have shot a great deal with the TL 120 on bright sunny days at f
19 and f22. Mostly handheld at 60th of a second - occasionally
1/30th. I get great depth of field from about 12 feet through to
infinity. I can only compare these shots to my old Sputnik. The TL
120 is sharp over the whole image whereas the Sputnik was patchy -
sharp in the middle but often fuzzy or distorted around the edges.
I'm finding the TL 120 very steady and well balanced in the hand
too, making it a very easy camera to shoot at 1/30th and 1/60th hand
held. I almost exclusively used the Sputnik on a tripod because of
many blurred hand held shots. I have also just taken a few shots at
f2.8 which I didn't think I would ever use but it came in handy
during the recent visit of comet McNaught.

I was happy with the Sputnik until I got my first roll back from the
TL 120. Now the Sputnik sits quietly on a shelf.

regards,

Harry Calderbank

Chuck Holzner writes:

> Does anyone have TL120 images shot at F/22? How are they compared to
> images out of older stereo cameras? It seems like most people who own
> them are shooting only the TL120 and so have no side by side comparisons.

Yes I do and I am out the door in a few minutes to get lots more *{;-)

I also have images I have taken with a Sputnik and a Don Lopp modified
Heidescop. A well-tuned Spud can take really good images, an untuned one
can take lousey images. I sold the Spud quite some time ago so can't make
any side-by-side comparrisons. My Heidescop takes really great images.
However it now has a broken shutter spring so I can't make any side-by-
side shots with the TL120. Once again, if anyone has a lead on a shutter
spring for this camera please let me know.

Comparing the Heidescop and TL120 images I have so far I would say they
are very comparable. What I mean to say is that both are very sharp over
the full frame. I can't really comment on comparable contrast and color
rendition as I don't have the same image from both cameras.

What I can say is that the TL120 is so much easier to shoot with than any
other medium format camera I have ever used. It is also the easiest to
compose with using the prism finder as compared to the unusable waist
level finder in the Spud and the modified Polaroid viewfinder Don made
for my Heidescop. Don's finder is very good and I highly recommend them
for Spud's and Heide/Rolliescopes though.

So what else do you want to see compared or know about the TL120?

For the record I haven't had any problems with my TL120 (version 3?) yet.
Nothing is broken, everything works. I will be carrying it in a backpack
shortly for a few miles. We will see how it survives that as well as
shooting in below freezing temperatures.

Thanks,

David W. Kesner

Chuck wrote:

>Matching the convergence is accomplished by matching
>the taking lens FL with the Viewer lens FL, the base to
>eye separation, and setting infinity parallax to zero
>(Parallel view to infinity).

This last week for me has been a process of thinking out loud at the
keyboard. I now have doubts about convergence matching and about setting
parallel view to infinity. As I said in my previous message, it is
difficult to sense the amount of convergence of your eyes. I suspect that
they might be more crosseyed when looking through a viewer than you
imagine. Considering the specifications of the 3D World viewer, your idea
does seem to make sense. I measure the separation of its lenses at about
64.5mm. Add the nominal on-film deviation of 2.5mm to the mount aperture
spacing of 62.0mm. and what do I get? 64.5mm. The eyes must be parallel
viewing distant objects in a stereo pair...end of story. But then I
consider my 35mm stereo slide viewer. It has the same lens separation as
the 3D World viewer and uses the identical mount aperture separation.
However, add the nominal 1.2mm on-film deviation to 62.0mm and you get
63.2mm infinity separation, well short of the 64.5mm you need to get the
eyes looking parallel. I tried remounting a 35mm slide to 64.5mm infinity
separation. Sure enough, it ended up uncomfortable to view. I also noted
how difficult it is to view magic eye stereograms. These facts led me to
suspect that the eyes are most comfortable viewing stereo pairs when
converged at somewhere near reading distance, and that they get
progressively less comfortable as they diverge towards parallel viewing.
The way to resolve this question definitively would be to actually measure
the convergence of the eyes of a person looking through a stereo viewer.
Can you think of how we might do that?

Oleg

Measuring eye toe-in seems like something that an eye doctor would be able to do, maybe. Seems like it would take some rather special equipment. The best I could do would be to measure the inter-ocular, eye spacing, and image spacing and try to calculate. It would take an adjustable inter-ocular viewer to allow the person to adjust for his relaxed toe-in.


I have researched the "Near and Far" dimensions some. In Ferwerda 24-5 he lists the only standard I can find (ISO 515) and it is for the Realist. It has 62.2 mm near, 63,4 mm far. Real 3-D Mounting Gauges by David Starkman has for Realist format, 62.4 near and 63.6 far. He has for Euro format, 62.1 near and 63.3 far. All with 1.2 mm OFD but over a range of .3 mm. 25%.

For slide mounts, I measure the heat seal at 62.1 and the RBT at 61.85

I only have two 35mm viewers that do not have inter-ocular adjustment. The Belcaskop meant for Euro format, and likely the Belplasca slide mounts, has an inter-ocular of 64 mm. My Slim 6X I bought from 3D Concepts has an inter-ocular of 62.4 mm.

What a mess. For 35mm viewing it is obvious that the only standard is now ignored. One could mount in RBT mounts and view in the Belcaskop with ( 64 - 61.85) 2.15 mm OFD with no problem, however mounting in the heat seals and viewing with the Slim 6X will give you only ((62.4 - 62.1) .3 MM OFD to play with. It is a good thing that most of us can take a degree or so of toe-out without pain and that most of the 35mm viewers have an inter-ocular adjustment. So much for 35mm stereo data. It is no wonder that people get headaches watching projected slides.

I know of no official standard for MF stereo. I did measure both the Spicer RMM mounts and the Q-Vue King Inn mounts to have 62 mm spacing. I read that the 3-D World mounts have 62 mm spacing as well. Wonderful. The Saturn is said to have 65.5 mm inter-ocular and the 3-D World viewer is said to have 65 mm inter-ocular. Using the 3-D World viewer with any of the mounts, you can get 3mm OFD behind the window and that is about all you can use and reasonably keep the DoF in limits at F/22. Of course you can get 3.5 mm OFD behind the window using a Saturn but it is nice to share your views with others who have the narrower inter-ocular. Those with adjustable inter-oculars can adjust. I would suggest a standard be adopted; 65 mm for infinity spacing, set the near by changing the aperture spacing on the mount if you get window violations. I have been mounting that way for years with good results.

I guess that people using a viewer and thinking that the view is in the little "box" will toe-in and adjust eye focus for close just before putting his eyes to the viewer and get very used to it. That should work well for near-sighted people not using their glasses. I would think that the parallax and near focus would lead to "Doll House" Effect. It will reduce the behind the window depth (OFD) that can be used without masking the mounts as the far spacing will need to be less to toe-in. I am far-sited and view without my glasses. I generally look to the distance and raise the viewer to my eyes without changing focus to view. Of course I have the viewer focus set to work for my eyes focused at infinity.

Parallel to infinity works well for me. it makes everything look the same size and at the same distance as it did in real life. I feel that is the way it should be. I have set inter-oculars far apart in 35mm viewers and the toe-in parallax makes me think everything is "doll house" size. I don't have an inter-ocular adjustment on a MF viewer and really don't want one.


Chuck





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Chuck,

Thanks for all the good data. Lots to think about. I fiddled around a bit
with my Realist Red Button viewer and can say unequivocably that there are
no "doll house" or gigantism effects due to degree of eye convergence. I
also changed my eyesight from near sighted to far sighted (by putting on my
glasses) and sensed no change in the psychological effect of the slide I
was viewing; once I refocused the viewer, the scene looked the same as
before. The viewer also enables me to force my eyes from a state of
extreme crosseyed convergence to moderate paneyed divergence in real time.
As I swing the interocular between those extremes the appearance of the
slide remains absoluley unchanged. The only change I can sense is the
amount of strain my eyeball rolling muscles feel. But I feel no pain at
either extreme. It seems that the eye muscIes and brain can adapt to
variations of convergence and interocular in stride. I continue to suspect
that any real pain while viewing stereo is caused primarily by vertical and
rotational misalignments and dimensional mismatches between the chips of
the stereo pair.

The Red Button viewer has a range of interocular from 59mm to 68mm and
feels most comfortable set at 65mm, whether I view with the naked eyes, or
my distance glasses, or my reading glasses.

I am still trying to get my mind around the "little box" effect. One does
not expect to see the whole wide world through a small hand held device.
As realistic as stereo views can be, it still takes some suspension of
disbelief to immerse onself in a view. That said, I did once manage to
fool a stereo novice into thinking he was looking at a real scene through a
pair of binoculars.

Oleg

Oleg wrote:

> I fiddled around a bit

with my Realist Red Button viewer and can say unequivocably that there are
no "doll house" or gigantism effects due to degree of eye convergence.


I think that once you have locked on to a scene, just moving the inter-ocular won't change the apparent size. It sometimes happens to me when looking at a slide of something I do not know beforehand and the convergence is way in. Convergence is the main "Depth Cue" for depth since we don't have sonar like bats.

> I also changed my eyesight from near sighted to far sighted (by putting on my

glasses) and sensed no change in the psychological effect of the slide I
was viewing; once I refocused the viewer, the scene looked the same as
before.


I would not expect adding or removing small amounts of magnification to change much as it all adds up about the same after you adjust the focus on the viewer. For people with young eyes, focus is a depth cue they may still be using, but not much at my age.

> The viewer also enables me to force my eyes from a state of

extreme cross-eyed convergence to moderate paneyed divergence in real time.
As I swing the interocular between those extremes the appearance of the
slide remains absoluley unchanged. The only change I can sense is the
amount of strain my eyeball rolling muscles feel. But I feel no pain at
either extreme. It seems that the eye muscIes and brain can adapt to
variations of convergence and interocular in stride.


Once locked on, I can see that the eyes would track and the appearance would not change much. It is the initial lock on that sometimes gives me the wrong size image.

> I continue to suspect

that any real pain while viewing stereo is caused primarily by vertical and
rotational misalignments and dimensional mismatches between the chips of
the stereo pair.


I don't see the vertical mismatch as a great problem. I need to view the top or bottom edge to detect vertical misalignment, otherwise it would have to be of a great magnitude. I see rotation along the top and bottom edge as a difference in vertical alignment along the edge. I would think that if both chips were rotated the same amount in the same direction a few degrees I would not notice it. I have at times leveled the horizon that way. I think we all get used to not having our heads "on straight" and often rotated to some degree.

>I am still trying to get my mind around the "little box" effect. One does

not expect to see the whole wide world through a small hand held device.
As realistic as stereo views can be, it still takes some suspension of
disbelief to immerse oneself in a view. That said, I did once manage to
fool a stereo novice into thinking he was looking at a real scene through a
pair of binoculars.


I still feel that, of all the depth cues we use, it is best to have them all in agreement as to what the depth really is, as much as possible. That is, if you want your scenes to look real and feel like you are really there. Parallax is a major cue necessary to stereo viewing and we should make an effort to match it to the view correctly.

Chuck




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Chuck wrote:

>Convergence is the main "Depth Cue" for depth
>since we don't have sonar like bats.

I guess we disagree on this point. In all the stereo pairs I have looked
at, the amount of convergence was never a factor as to how I perceived a
scene in terms of space. It might be that that others are sensitive to
this, but I am not. Like I said before, I suspect that, if you could
measure the convergence of your eyes when viewing stereo pairs, you would
find that they are converged even though they feel parallel. This is
certainly true when you are viewing most 35mm slides, since viewer lens
spacing is typically so much greater than the infinity spacing on the chips
(your Slim 6x viewer excepted). I already mentioned how uncomfortable a
35mm slide became for me when its distant subject matter was set for
parallel viewing.

>I don't see the vertical mismatch as a great problem.

It is not as bad as rotational mismatch, but by the time vertical mismatch
hits 2%, my eyes are tearing.

>I would think that if both chips were rotated the same
>amount in the same direction a few degrees I would not
>notice it. I have at times leveled the horizon that way.

Agreed. I frequently do the same. In a folio I once sent out a slide in
which I leveled a two degree tilt. I requested comments, and no one found
it difficult to view. If you think it through, this method of leveling
boils down to a vertical mismatch in nearby subject matter. As such, it
becomes uncomfortable when it breaks the 2% rule I mentioned above.

>Parallax is a major cue necessary to stereo viewing and
>we should make an effort to match it to the view correctly.

Again, we disagree on this point.

Oleg

Of the dozen or so depth cues in photos only two are not present in a flat photo that are present in a stereo photo. Those two are:
1) the ability to see around the left side of an object a little farther with the left eye and around the right side a little farther with the right eye. With that depth cue left out we can call it "Card boarding" but we still see depth from parallax. If we take the parallax out we will have little perception of depth over that of a single image. (I really don't know how to take parallax out and leave #1 in.)
2) Parallax shows us depth by giving us different toe-in angles, more for close and less for far objects. It is the means used in many cameras for the range-finder. The parallax can be off from the normal "parallel to infinity" and you can still see depth as the parallax is still more for the closer objects and less for the far objects.

Knowing how big something is also helps figure out the depth. That is one cue we use in flat pictures or we would be saying "boy that car sure is small and those people are only a couple of inches tall." I remember seeing a stereo view of a key collection in a folio a few years back. It was a view of several antique keys. The keys were mostly of the old "Skeleton Key" type and different sizes, The more I looked at them the less I could determine their size, at times they looked to be only 2 or 3 inches long (as I remembered that type key from my childhood) and at other times they looked to be as much as 2 feet long. I didn't try to measure OFD or homologous spacing but it became obvious to me that I was receiving mixed depth cues.


Oleg Wrote:
Like I said before, I suspect that, if you could
measure the convergence of your eyes when viewing stereo pairs, you would
find that they are converged even though they feel parallel. This is
certainly true when you are viewing most 35mm slides, since viewer lens
spacing is typically so much greater than the infinity spacing on the chips
(your Slim 6x viewer excepted). I already mentioned how uncomfortable a
35mm slide became for me when its distant subject matter was set for
parallel viewing.

Chuck:
I can mount for parallel infinity viewing with MF and not feel discomfort looking near or far in the view. I can do the same for 35mm and still feel no discomfort. I don't know how you are measuring the inter-ocular of your 35mm viewers. In a search through my stereo equipment I did find two more viewers with fixed inter-ocular. One is a cardboard viewer I got from Jon Golden and the other a Radex-gem I bought with a camera off E-Bay. I measured The inter-ocular of each by using the "point the lenses at the sun and measure the focused dot spacing" method. Both of them have inter-oculars in the 62.5 to 63 mm range. I can't measure them any closer than that but neither has 65 mm inter-ocular as you say yours has. What viewer are you measuring and how? You can't assume that the lens optical center is in the center of the lens holder.


Chuck wrote:

>I don't see the vertical mismatch as a great problem.

Oleg wrote:

It is not as bad as rotational mismatch, but by the time vertical mismatch
hits 2%, my eyes are tearing.

Chuck:
That would be 1 mm offset left to right image with the 50 mm vertical in MF slides. I only find that a problem when looking at the top or bottom edge of the view. I doubt that the lenses FL in the viewer are checked to be matched to less than 2% at the factory. I have seen slides that were shot with cameras that have about 2% mismatch in the lens FL and looked good except that the vertical alignment was right on at the bottom of the slide and off at the top. Let me say that I do try to mount with zero vertical offset.

Chuck:
Parallax is a major cue necessary to stereo viewing and
we should make an effort to match it to the view correctly.

Oleg:
Again, we disagree on this point.

Chuck:
I can handle the disagreement. %^)

Chuck




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Chuck, I’m not sure what you are describing in #1. The “seeing around” is
the result of the “parallax view” from two viewpoints. I think that
“cardboarding” is the result of low resolution, high contrast and/or removal
of 2D clues to depth and roundness.

On #2, “toe-in angle” seems like you are referring to convergence. I think
convergence is the very weakest contributor to depth effect. Parallax
“differences” recorded in the image are responsible for almost all of
the stereo effect and the trump card in 3D.

Regards, LeRoy

On 2/8/07 4:32 PM, "Chuck Holzner" wrote:

Of the dozen or so depth cues in photos only two are not present in a flat
photo that are present in a stereo photo.  Those two are: 1) the ability to
see around the left side of an object a little farther with the left eye and
around the right side a little farther with the right eye.  With that depth
cue left out we can call it "Card boarding" but we still see depth from
parallax.  If we take the parallax out we will have little perception of
depth over that of a single image.  (I really don't know how to take
parallax out and leave #1 in.) 2) Parallax shows us depth by giving us
different toe-in angles, more for close and less for far objects.  It is the
means used in many cameras for the range-finder.   The parallax can be off
from the normal "parallel to infinity" and you can still see depth as the
parallax is still more for the closer objects and less for the far objects.