T3D Re: 30:1

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

John Bercovitz wrote:

>>A translation of the object by 1/30 of the field of view should
>>> also work. 

>  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. 

That's correct.  The effective focal length is equal to the
working distance times the magnification.  "Working distance"
is the distance from the lens to the subject... this can vary...
for high resolution work is made shorter... let's say it is
10 mm and mag is 100x then we have FL = 1000 mm... quite long
and can be much longer at higher magnifications.

>I wonder what 1/30th of the field size would be like.  

Let's assume that you get a 4" print at 100x magnification.
Then the field of view is 100 mm/100 = 1 mm.

So Field of view is 1/1000 of FL... John was right!

An approximate formula is FOV = 10/MxM FL (M=magnification)
for M = 100 then FOV = 1/1000 FL but for M = 1000 then 
FOV = 1/100000 FL.... so now you know why simple translation
will not work...

On the other hand, tilt is perfectly acceptable... There
will not be any keystone distortion at moderate magnifications
because of the very long effect focal length of the instrument.

George Themelis


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