Stereo
Microscope
Magnification
Field of View or
Field Diameter
Many people ask the question
"How much magnification will I get with a 10X or 30X low
power (stereo) microscope?" As a result, we took
some shots of a penny at various powers. What you see below
is the approximate field of view and relative
magnification that you will see when using a low
power or stereo microscope. Approximate field of view is
the distance (from left to right) that you should see in your
eyepiece and relative magnification is the size
relationship between the images.
Magnification is a relative
term, especially in this day of digital and video imaging.
For example, a penny is actually about 20mm wide (diameter).
If you took a metric ruler and measured the size of the penny
image below, you would get a larger number (depending on your monitor size
and characteristics, let's say you measure 10cm or 100mm). The amount of
magnification is then the ratio of those two numbers. In our
example, 100 divided by 20 is 5X. If I displayed this same image
on a wide screen projection TV then I could make the penny much
larger, say 1 meter wide (1000mm). Now the magnification
would be 1000/20 or 50X.
So, the power you should choose
is dependent mainly on your applications. Find out how
big your specimens will be and think about how much of the field
(field diameter) you will want to see at one time
Field of View
at 10X
Total magnifying power 

Field of View
at 15X
Total magnifying power 

Field of View
at 20X
Total magnifying power 

Field of View
at 30X
Total magnifying power 

Field of View
at 40X
Total magnifying power 

Learn
more about Field Diameter here