P3D Bruce's Challenge - II

["Dr. George A. Themelis" <DrT-3d@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

So, Bruce has challenged us to show how changing the focal length
(F) and the stereo base (B), affects the recorded and the perceived 
stereo image.

Note, I made a distinction between "recorded" and "perceived" image.
My previous posting dealt with how these three variables (F, B, I)
affect the recorded image.  These are things you can measure with
a ruler.  But how about the perceived image, i.e. the stereo image
that we experience during stereoscopic observation?

To understand what we see we need to know two more things:  The
ocular spacing of the observer (that's the distance between the
eyes - not so easy to change!), Bv.  And the focal length of the
*viewing* lens, Fv. (If a lens is not used then we need the viewing
distance).

Stereo photographers have readily identified this condition:

B = Bv & F = Fv 

as ORTHOscopic STEREO.  If the stereo base is equal to the spacing
of the eyes (roughly 65 mm) and the focal length of the recording
lens matches that of the viewing lens then we perceive the stereo
image in a way that imitates reality (viewing with bare eyes) as
close as possible.

Standard 35mm stereo cameras are designed for ortho stereo.  For 
example, the Realist has 70mm lens separation (close enough to 65mm). 
The focal length of the recording lens does not matter as long as it 
is matched by the FL of the viewing lens.  Most 35mm film viewer 
lenses have a FL between 40 and 50mm.  The choice of 35mm FL for
the Realist camera is a compromise giving near-ortho conditions
with good depth of field and wide enough field of view.

If the ortho stereo conditions are not met they we can distinguish
the following situations:

B >> Bv: Hyperstereo
B << Bv: Hypostereo

F >> Fv: Stretch
F << Fv: Squash

To be continued....

George Themelis