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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 (in green
below), 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? (answer at
bottom of page) |
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. |
