Re: [photo-3d] Apparent vs. actual focus

["John A. Rupkalvis" <stereoscope@xxxxxxxxxxxxx>]

This is a very good and apparently accurate commentary on the way that many
people perceive depth on the projection screen.

Our perception of depth, and depth relationships is very strongly influenced
by the characteristics of the subject matter and our logical expectations.
This is especially true in the absence of any "real world" depth references.

For example, if the auditorium is totally darkened, and the seating is such
that we do not have other references such as other people's heads in the
audience in front of us obscuring the bottom part of the screen, we may have
to actually remove our 3-D viewing glasses to make an accurate judgment of
the actual distance of the screen.

This makes judgment of such factors as when subject elements are behind, at,
or in front of the screen plane, difficult and often erroneous.

I became personally aware of this by an event that took place some time ago.

I had seen some demonstration footage at a screening room, along with a
friend who had also attended the same screening, but was seated in different
location.  There were some other people seated in front of me, but no one at
all in front of him (he was seated directly behind an audio board that was
only about 2 feet high, so there were not even any chairs in front of him).

I was quite impressed by how far objects appeared to come off of the screen,
sometimes even passing the heads of the people seated in the row in front of
me.

After the screening, we compared notes.  He commented on how nice the 3-D
was, and that he was pleased that they used restraint in filming it such
that everything was behind the screen, and nothing came into theater
space...

I was so surprised by this statement, that I suggested that we both see this
same film again at the next screening, which was scheduled for the afternoon
of that same day.

We did, only there were fewer people at this showing, and no one was seated
in front of either of us.

My reaction was that although the film looked essentially the same, the 3-D
did not appear as dramatically far off the screen as it did to me earlier.
And, he was still maintaining that everything was behind the screen plane.

Just to make sure I wasn't fooling myself, I looked over my viewing glasses
and confirmed to myself that there was both positive and negative parallax
in the particular scene we were viewing at the moment, proving that some
objects in the scene were indeed in front of the screen.

Then something happened.  A person who had been seated in the row in front,
but way off to one side, got up and walked to the other side of the room,
crossing directly in front of us.

At that point, my friend exclaimed: "OH, MY GAWD!!!".  I said: "Are you all
right?"
He said: "I think so.  But that pole (in the actor's hand in the scene) went
right past that guy's ear" (the fellow in the audience who crossed in front
of us).  He continued: "It nearly poked me in the eye!"

Actually, I feel that my friend was being overly dramatic.   The actor was
near the convergence point (again double checking by looking over my
glasses), and the pole, held at about the center point, was in actuality
only 6 feet long (I had seen the actual prop pole on the set, my friend had
not).

However, my friend did admit that objects actually were in theater space.

Something to think about.   I am curious if anyone else in the group has
ever had a similar experience.

JR

----- Original Message -----
From: "John Goodman" <jgood@xxxxxxxx>
To: <photo-3d@xxxxxxxxxxx>
Sent: Friday, October 20, 2000 7:13 AM
Subject: [photo-3d] Apparent vs. actual focus


> I've been exploring stereo projection, and can't quite pin down
> what my eyes are doing in terms of focusing. I see the screen
> acting as a window into the scenes, but the "depth" of the views
> beyond the screen plane appears to be about equal to my
> distance from the screen. For example, when the projector is
> 10 feet from the screen and I'm viewing the screen from six
> feet away, the perspective of the stereo view seems to extend
> about six feet beyond the plane of the screen regardless of the
> image content, no matter if the infinity points represent objects
> that were 500 feet or 50 feet distant or, in the case of a macro,
> five inches. Of course the details of the scene make the
> interpretation of what I see appear pleasingly realistic.
>
> For increased viewing sharpness, I've been wearing prescription
> glasses behind polarizing glasses, and these have progressive
> focus lenses, from a near two feet at the bottom to infinity
> toward the top. As far as I can tell, regardless of the apparent
> depth of the stereo view, I'm focusing at or slightly beyond the
> screen plane, which makes sense, correct? What does theory
> say about this?
>
> Thus there are usually major differences between the plane of
> physical eyesight focus during stereo projection and the planes
> of focus that existed in the photographed scene, not to mention
> corresponding disparities in convergence. I imagine that our
> visual system is adept at favoring the most apparent depth cues
> while ignoring conflicting factors. And experience increases
> one's ability to both create the illusion of realistic 3-D space
> while ignoring conflicting aspects?
>
> John Goodman
>
>
>
>