All I know about the SEM (very short)

[T3D Dr. George A. Themelis <DrT-3d@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

Guys, guys, guys... I cannot participate here by reading the digests...
By the time I read something and try to reply, later down the digest
someone has made a better point...

Is the SEM orthogonal or central projection?  It is central projection.
The center is at a "fixed" controlled distance from the subject known
as the working distance.  But, the key point is the magnification!
If the magnification is low and the working distance small, then you 
cannot assume orthogonal projection.  Tilt will introduce keystone
distortion just a a regular tilt with a camera.

However, at higher magnifications, only a tiny area is being "imaged"
(scanned).  In this case, you can safely assume orthogonal projection.

Translation will not work in high magnifications (practically anything
around 100x or higher).  That is because sufficient translation to obtain
any useful on-film deviation will get you at a totally different field
of view!  The only way to use translation with the SEM is at the lowest
possible magnification and smallest working distance.  You will still
lose part of your image as in aerial photography.

One way to simulate the SEM is to think of a camera with a lens whose
focal length is equal to the working distance of the SEM, times the
magnification.  For WD = 20 mm and Mag = 1,000, this leads to 20,000
mm focal length (quite long lens, I would say!)  Imagine taking
pictures with a 20,000 mm fl lens of something sticking off the 
wall... What is the field of view of such  lens?  Pretty small!  
You translate 1 mm and you lost your image.

John B. asks about the impression of viewing SEM stereo pairs and
the consequence of the "infinite fl" or absense of linear perspective
in the SEM shots.  John, yes, they should look unatural because they
lack linear perspective.  Similar distortion results when taking pairs 
with lenses with long FL.  Points which are far away are not smaller
because distance does not decrease sizes.  You can create pairs
of cubes and eliminate perspective and use only parallax.  The
effect is the same.

Regarding the 6 degree tilt that supposedly gives you a natural
impression, this is BS in my opinion!  Since the situation is so
unatural, only the amount of maximum deviation should be used
to determine the tilt.  This will also depend on the overall
depth of your specimen.

That's all I know about the SEM (which is not much), without using
a single formula. -- George


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