P3D Re: Hypo - Ortho - Hyper OR convergence angle?

[Larry Berlin <lberlin@xxxxxxxxx>]

>12-4-97
>Dr George Themelis writes:
>...........................
>>Larry asks:
>>
>>why pay all this attention to the terms Hypo, Hyper, and Ortho at all? 
>
>Because they are derived from Greek roots?  :-)
>
>Hypo:  under
>Hyper: over
>Ortho: correct

****  So have the Greeks no term for angular measurements or ratios? I have
no argument with the terms. They mean what they do and no more than that. 

>
>These terms just cover general situations.  They give us some idea of 
>the subject matter and the way it was recorded.  

***** That's the reason for my comment. The ideal angular convergence which
is defined by the ideal Ortho stereo base drives the choice of any other
*ideal-per-scene* stereo base from Hypo to Hyper and extreme Hyper. However
the terms Hypo and Hyper only indicate a direction of difference without a
hint of the amount. Therefore they convey extremely little information about
how a scene was recorded. Many Macros and Micros couldn't exist if they
weren't Hypo yet calling them hypo says little about whether the image looks
good or bad or was taken with a depth exageration, or taken so close
together that they look flat. A convergence angle would convey information
about whether the image has apparent depth. Or would help someone else
repeat the circumstances for themselves should they wish to.

Suppose I described a picture of a flea as a hypo, what do you expect? How
do you react when you see a picture taken with a stereo base of 1.75 inches
at a distance of 5 feet with a 35mm lens of a small dog named *flea*? Yet
that is as accurately *hypo* as an SEM of an actual flea. 

Having spent a lot of time getting stereo shots with a single camera, it's
almost painful to use a fixed base stereo camera. It's like tying a string
of maybe 14 inches length between your ankles and trying to walk fast. So
many of the subjects I'd like to photograph are just a little ways out of
the typical *ideal* range for a stereo camera. With a single camera, I
change the base distance freely with each subject. So easy and so free
feeling and so hard to quantify according to hypo/hyper. With the fixed base
stereo camera too many subjects are *out of bounds* by being too close or
too far for ideal results. I still like using stereo cameras though! :-)

>
>How about the terms "wide", "normal" and "telephoto" used to classify 
>lenses?  These seem arbitrary to me too.  

*****  Not really arbitrary, just generalized. When one has a zoom lens, one
usually describes the fl used for a scene. Saying they used Zoom, wouldn't
describe which end of the range they used and telephoto might be considered
anything between 50mm and the maximum available in each instrument.
Similarly it would make sense to describe the convergence angle for a scene,
describing by that means a more accurate story of how the picture was taken.
It would tell you a lot about what to expect in the viewed results. 

The ideas of the 1:30 rule or the 1:15 rule are another way of describing
the convergence angle by expressing the ratio of distance to baseline. This
information should get more weight in common descriptive usage than the
terms Hypo and Hyper.


>>(I said) ............................ to 
>>appear with satisfying depth relationships.
>
>Hmmm... I don't agree with this statement... "Satisfying depth"?

******  That's a subjective term. You apply it according to your own rules.
There must be images that you like more than other images, relating to
observable depth factors, along with the assumed compositional aspects. In
essence this differentiates between 2D flatness and stereoscopically
presented 3D. You must have some place along that scale where you draw the
line for your own personal preferences? Most people that I know seem to have
such a preference border. That definition would describe one limit of a
broad range of acceptance factors.

>
>Let's see... A subject at 7 feet (2130 mm) is at a convergence angle of
>2* invTan(70/(2*2130)) [70 mm is the lens spacing in the Realist]
>which is 1.9 degrees.  That's a small angle.  My SEM stereopairs are 
>recorded using at least 6 deg. angle tilt.  Yet both appear to have 
>"satisfying depth relationships".  

*****  So both are within your acceptance range. Yet there is some angle
where the same subjects wouldn't be acceptable to you. That's why you chose
the ones that you did. Your SEM pairs are Hypo by definition. What happens
if you used 6 degrees of rotation, identical to how it's applied in the SEM
situation, with a standard camera and human subject?

Will it be satisfying or too exagerated? Will it be Ortho? I doubt it. I
would expect that it would fall under the general classification of Hyper
instead of Hypo. Describing the SEM as a Hypo just doesn't go very far.
Describing it's convergence angle gives you something of an idea of what to
expect in the resulting view. If it's an SEM we kind of assume it must be a
Hypo, so why bother with that particular label in such an instance?

I would guess that with your SEM work, an angle of 0.005 degrees would not
satisfy you if you expected a stereo image. Yet it's as valid a quantity
relative to the term Hypo in these circumstances as is 6 degrees. 


>........................ I tend to believe that the
>only thing that counts for non-ortho situations (excuse me but I have
>to use this term to mean situations that cannot be seen with bare
>eyes) is the on-film deviation.  This needs to be restricted.
>Different subjects will require different convergence angles to
>give the same on-film deviation.

*****  That's a good example to use. It's a valuable dimensional bit of
information, but how to relate to it? You can't get in there to the film and
measure where the light hits the film before taking the picture, so you have
to go from experience. In the field, there is nothing about the camera or
your suroundings that indicates what is happening inside the camera relative
to this factor. Only experience and study of the images _after the fact_ of
taking them can indicate what is going on. In order to apply that knowledge,
one still relates it to more real-world measurable elements, like stereo
base and subject distance and focal length. That's what you are dealing with
at picture taking time.

>..............
>OK.  You can classify your images according to their convergence 
>angle.  I will continue to use the terms "Hyper", "Ortho" and "Hypo" 
>that give me a better idea of the subjects and the tools used to 
>record them.

The terms by themselves can convey nothing of the applicable tools or
techniques you are using to anyone but a person already familiar with your
situation and preferences. I didn't suggest that you or anyone else *not*
use the terms, just that the usage they seem to get so often is a bit
over-rated. (I wasn't pointing a finger at anyone) :-)

Larry Berlin

Email: lberlin@xxxxxxxxx
http://www.sonic.net/~lberlin/
http://3dzine.simplenet.com/


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End of PHOTO-3D Digest 2436
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