T3D Re: 30:1

[John Bercovitz <bercov@xxxxxxxxxxx>]

>> A translation of the object by 1/30 of the field of view should
>> also work.  Translations should work better than tilt, but hey, 
>> what kind of vacuum stage do you have?

I guess what we are talking about here is coverage of
a typical electron microscope, is that right?  I have
heard that they have extremely long focal lengths 
compared to field size.  In other words, it's sort of
like taking a picture through a high-powered telescope.
Perspective is essentially infinite.  So you can never
view from the correct distance; you will always be too
close because if you are at the correct distance, the 
images will occupy an extremely small piece of your eyes'
field and you won't be able to see nuttin'.

In photographic work, a rule of thumb is 1/30th of the
distance from the camera to the object.  That's different
from 1/30th of the field diameter.  I don't think you could 
translate 1/30th of the perspective distance in an electron 
microscope since its field is so small or its focal length 
so long.  I wonder what 1/30th of the field size would be 
like.  Has anyone here thought about this before?  Stream 
of consciousness follows.  If the field is 1/50th of the 
focal length, then a shift of 1/30th of the field is one 
part in 1500 of the focal length.  If the field is 1/100th 
of the focal length, then a shift of 1/30th of the field 
is 1/3000th of the focal length.  These are very small 
shifts.  Useful stereo extends out to as much as 1/100 and 
these figures are beyond that.

OK, question, what is the focal length and field of an
electron microscope?  I realize these are variables but
then what is the range?

Thanks,
John B


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End of TECH-3D Digest 491
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