P3D PePax challenge # 2

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

We have talked about long focal length lenses and lack of depth if 
these are used with a normal stereo base, leading many times to a 
"cardboard cutout" impression.  

At this point we should emphasize that it is not the focal length of 
the recording lens that maters but the relative values of recording 
and viewing lenses.  For example, if a scene is recorded with 135 mm 
lenses (in 35 mm film) and then viewed with a viewer with 135 mm 
lenses, we should see just a small orthoscopic view, equivalent to 
viewing a scene that was taken and viewed with 45 mm lenses and 
cropped (masked) down.

In a scene recorded with 135 mm lenses and normal base and viewed with 
45 mm lenses we will notice an apparent lack of depth.  That's because 
it appears as if we are closer to the objects photographed but the 
amount of depth is reduced because we are really not close to them.  
To increase the depth we can increase the stereo base.

Lets use 135 mm lenses (3X the FL of viewing lenses) and 3X the stereo 
base, 7.5" instead of 2.5" (200 mm instead of 65 mm).  Now, we are 
getting closer to fooling the brain:  The size of objects tells the 
brain that we are closer AND there is more depth as if we were closer.  
That's the PePax principle.

What's the catch?

By increasing the size and stereo base we are fooling the brain to 
thinking that we are really closer to the scene, *except* for the 
relative size of near by and distant objects:  Near by objects appear 
smaller than far away objects or far away objects appear larger than 
near by objects.

Let me try and explain this key point:  If are really close, then 
something at distance x to the camera would appear twice as large as 
something at distance 2x.  But, we are not really close, we just use 
longer FL lenses.  We are far away, trying to fool the brain by 
presenting a larger image and more depth.  Objects which *appear* to 
be at x and 2x from the camera, might really be 4x and 5x so they have 
about the same size.  The brain does not know that.  Expecting the 
close-by objects to be proportionally larger, the brain assumes that 
the further objects are larger than they are in reality (or the nearby 
objects smaller).

This is the only catch.  When is this a problem?   An example follows.

George Themelis


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