From: bercov@bevsun.bev.lbl.gov (John Bercovitz)
Newsgroups: alt.3d
Subject: Re: Enhanced parallax
Date: 16 Sep 1992 00:33:09 GMT
Organization: Lawrence Berkeley Laboratory, California
NNTP-Posting-Host: 131.243.192.10

In article <CB.92Sep15173111@tamarack13.timbuk> cb@tamarack13.timbuk 
(Chris Brewster) writes:

>Matthew Kasdorf writes:
>>   The [View-Master Personal Stereo Camera] works fine,
>>   I've recent picture of the Grand Canyon and Baja to prove it; ...

>Here's something about stereo photography that I haven't seen much
>discussed.  (I'm new here, so forgive me if this has been covered.)

>Although I've seen many different values for the maximum
>viewer-to-object distance at which stereoscopic perception can occur,
>the maximum is surely less than, say, a mile.  That is, I doubt that I
>could discriminate the distances of two objects at 1 mile and 1.5 miles
>solely on the basis of a 2.5-inch parallax.  For views of distant
>vistas, Viewmaster slides traditionally include something in the
>foreground to force your awareness of depth.  But there's another
>approach.

I like to think of 20 feet (6 meters) as being a good maximum.  If you
effectively make your farthest subject of interest that far away, you'll
have great depth.

>I have two aerial photos of Manhattan taken around 1920, which were
>apparently taken from a balloon.  As I reconstruct the technique, it
>appears that the photographer took one shot, let the balloon drift maybe
>50 feet, and then took the other shot.  The left and right shots show
>evidence of 10 or 15 seconds interval between the two shots.  The result
>is an artificial depth perception that one couldn't see in real life,
>very much like looking down on a highly detailed table model.  For
>reproducing how you would actually see, this is the wrong approach, but
>if you think stereo photography is a way to add information to a photo,
>this technique has great value; I think more should be done with this
>approach.  Does anyone have experience with this?

I think hyper is one of the most exciting aspects of stereo photography.
I've photographed the Grand Canyon with separations of hundreds of yards
and local hills with only a hundred yards.  As you say, it allows you
a view which is not available in nature, a "giant's view".  There's an
incredible amount of information there.  It's like using the displacement
parallax you get in a fast-moving car only much, much better.

The way I figure it, if the far hills in a shot are two miles away, that's
10,000 feet or 500 times the desired far distance of 20 feet.  500 times
a 2.5" inter-pupillary distance is 35 yards.  So 35 yards would be a sort
of minimum distance between cameras if you want to have the hills two miles
away well separated from the infinite background.

You can easily get by with only one camera in a nature shot.  But when I
take a pair of the city by night from the hills of the east bay, I must use 
two cameras or suffer the annoying distractions of changed traffic lights
and displaced cars to name a few.  In a nature shot, you do have to look
out for the shadows of moving clouds.

Center each camera on an object at infinity to avoid any hint of keystoning.
Keystoning is not generally a very big problem in this type of photography.
Look out for foreground stuff; you don't want any.

If you use two cameras, you probably won't have to worry about lens focal
length matching if you use two high-production lenses.  The local authorized
Canon repair station in Hayward very kindly let me test all of their 50 mm
focal length f/2.8 Canon FD lenses.  These lenses were anywhere from around
20 years old to brand new.  The focal lengths didn't vary much at all.  I 
don't recall the exact figure but it was well under 1%.  To test focal length,
just set your camera on a tripod, take a picture of something that will put
a couple of marks on your negative at the extremes of the negative, then
swap lenses and do it again.  When you get the negatives back, just measure
the distance between the marks with a precision ruler and a high-power 
magnifier.  If both lenses give the same distance between marks, you're in.

I like to use normal lenses.  I don't see a particular advantage in
using long or short lenses as you may get compression or extension of the
object in the axial direction (to and from you).  I'm assuming here that
your viewing lenses have some similarity to the focal length of a normal
lense.  I certainly wouldn't get wilder than a factor of two different
between the lengths of the taking and viewing lenses.

For the two-camera set up, I have one camera with a motor drive which allows
a remote cable to activate it.  You don't have to be dead on in matching the
time of each shot (unless there's an airplane in the picture) but there's sure
no harm in getting them to fire as close to the same time as is possible.

For closer shots, mounting the two cameras on a bar works well.  But I'll 
leave that description up to John Roberts who has done quite a bit in this 
area.  8-)

John Bercovitz     (JHBercovitz@lbl.gov)

Newsgroups: alt.3d