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Field of View
or Field Diameter is very important in microscopy as it is a
more meaningful number than "magnification". Field diameter
is simply the number of millimeters or micrometers you will
see in your whole field of view when looking into the eyepiece
lens. It is just as if you put a ruler under the microscope
and counted the number of lines.
The chart below
will tell you (approximately) what to expect when looking
through a microscope with varying combinations of eyepiece and
objective lenses. As an example a dual power stereo
microscope with 10X eyepiece lenses and 1X and 3X combinations
of objective lenses, would have total powers of 10X and 30X
and your field of view would be 20 mm and 6.7 mm
respectively.
This means that
an object 20 mm (2 cm, or about 3/4 inch) wide would fill up
the whole viewing area at 10X and an object about 6.7 mm wide
would fill up the whole area at 30X. As you can see, having
the highest power may not be best for your particular
application. When you move to greater magnifications, you
sacrifice field of view.
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Try this
Get a metric
ruler and place it on the stage of your microscope.
Illuminate from above (if you are using a compound
microscope, get a transparent ruler or illuminate it
with a flashlight). See how many millimeters you can
see from left to right. What would be the field of view
of this image? |
Other
considerations: The working distance is the distance from the
bottom of the microscope (lens) to the part of the specimen
that is in focus. As you increase the magnification, you
decrease
the working distance. If you need to work under the
microscope, you will need a large working distance. Some
special microscopes have extended working distances for these
purposes. Zoom microscopes have a fixed working distance
throughout the zoom range. When using a 100X objective lens
(1000X total power) your working distance might only be 0.04mm
(40um). The lens will be extremely close to the specimen!
The working distance and the amount of vertical motion of the
microscope will also affect the maximum specimen height.
Maximum Specimen Height is how tall an object you can put on
the stage and still be able to focus on the top part of the
specimen. |
