USING THE MICROSCOPE TO ESTIMATE SIZE
1.
The Microscope
Name:
There are 2 sets of magnifying lenses:
eyepiece/ocular and objective. Each set of lenses
contributes to the total magnification:
Total magnification = eyepiece
magnification x objective magnification
Rotate the nosepiece to change objectives—this
changes total magnification.
The eyepiece does not change and contributes to
the total magnification for all objectives used.
2.
Your microscope magnifications: Record the information for your microscope in the table below.
Record the whole number
found on the eyepiece/ocular.
This number will be the same
for all microscope powers.
Microscope
power
Eyepiece/ocular
magnification
Record the whole
number found on
each objective.
Objective
magnification
Total
magnification
(see formula above)
low
medium
high
Every time you need to estimate the size of a specimen you must use the same microscope.
My microscope number is:
3.
Looking at stuff and estimating size. When you look through the microscope you see a
circle of light; this is called the field of view (FOV). If you know the FOV diameter in mm (or
other units), you can use this to estimate the size of any specimen:
Example: if the diameter of the FOV is exactly 1 mm, then you would
estimate that the size of the letter “e” is 1/3 mm or 0.33 mm, since it
looks like three of the letter “e” would fit across the field diameter.
Field of view (FOV)
Field diameter
 To estimate size measure the field diameter of the FOV for your microscope, then use this
measurement to estimate the size of the specimen.
If you look at a letter “e” under the microscope, you will notice two things:
1. The image of the letter “e” is flipped both upside down and sideways.
2. When you move from low power to medium power to high power, the image of the letter “e”
gets bigger, BUT THE FIELD OF VIEW GETS SMALLER (the area that you can see
shrinks). This is shown below:
low power
medium power
high power
This means that you will need to find the field diameter for each power (three field diameters
altogether) so you can use any microscope power to estimate size.
The next section tells you how to do this.
4.
REALLY IMPORTANT INSTRUCTIONS FOR USING THE MICROSCOPE
Foolproof focusing rules:

Stage down, low power: Make sure the stage is down completely and the low
power objective has been selected.

Focus large: Place your slide on the stage and focus using the large, coarse focus
adjustment knob. With the low power objective, you do not have to worry about
crushing the slide.

Centre: Centre the specimen of interest.

Switch: If necessary, switch to the medium power objective. The specimen should
still be in view, and should almost be in focus.

Focus small: Use the small, fine focus adjustment knob to focus the specimen. If
you use the coarse adjustment knob, you run the risk of crushing the slide!

5.
You must follow these steps every time you use the microscope; you will
not damage the microscope or crush the slide and you will always be able
to find the specimen.
If necessary, switch to high power and use the fine focus to sharpen the image.
Measuring field diameter. Obtain a clear mm ruler from the teacher; use the rules above to
focus on the mm lines of the ruler at LOW POWER.
Move the ruler so that it is measuring the diameter of the FOV and that one of the mm lines
is right against the left edge of the field.
Draw exactly what you see in the FOV circle below.
The field diameter of the low power FOV is:
mm
use
Microscopic things must be measured in units
that are smaller than mm. The correct unit to
use is the m (micrometer).
There are 1000 µm in one mm (or, one µm is
1/1000 of a mm). To convert the field diameter
from mm to µm, multiply by 1000.
The field diameter of the low power FOV is:
µm.
LOW
Transfer the total magnifications you calculated on page 1 into the table below; as well, transfer the
low power field diameter (in µm) you just measured.
Relative
magnification
Total
magnification
(see formula above)
Field diameter
(in micrometers, µm)
low
medium
high
 Show this table to your teacher; they will give you the medium power field diameter and high
power field diameter for your microscope.
6.
Putting it all together; estimating the size of a specimen.
 Obtain a prepared slide of a laser-printed shape. Using the foolproof focusing rules, draw the
specimen in the FOV circles below. Make sure that your drawings are as accurate as possible.
Before you draw, move the slide
so that the specimen is against
one edge of the FOV.
LOW POWER
MEDIUM POWER
Now that you have drawn your specimen, you can use your drawing and field diameter measurements to
estimate the size of the specimen, using the following example:
LOW POWER FOV
Draw a line across the diameter of your field and through
the specimen. This line will help you determine how many
specimens fit across the field diameter (the fit number).
Use the following size is formula to calculate specimen
size:
Size = field diameter ÷ fit number
Make sure you use the field diameter that matches the
microscope power. In this case it is LOW POWER.
Assume my LOW POWER FIELD DIAMETER = 2900 µm
I estimate that the FIT NUMBER = 3.5 (this is the number of specimens that fit across the diameter)
size
= field diameter ÷ fit number
= 2900 µm ÷ 3.5
= 828.57 µm
Since this is an estimate, you should round off the answer to 830 µm. So, the
smiling fat cat is 830 µm wide.
Summary of steps for calculating specimen size:






draw the FOV (use a culture dish); indicate the power
position the specimen against one edge of the FOV
draw the specimen in correct proportion to the FOV
draw a line across the diameter and across the specimen
estimate the specimen fit #
calculate specimen size using the formula
Now, use example above to complete the calculation of size for the laser-printed
shape:
LOW POWER
MEDIUM POWER
field diameter =
field diameter =
fit number =
fit number =
show size calculation:
show size calculation:
FAQ:
Why did I draw the specimen as accurately as possible, in correct proportion to the FOV?
If you do not draw your specimen in correct proportion to the FOV, your fit number will
end up being incorrect. This will affect the accuracy of your size estimate.
Why did you position the specimen against one edge of the field diameter?
This allows you to estimate fit number more accurately.
How do I know which power to use to estimate size?
This is an important question. Use the power that allows you to see all of the specimen
and/or that gives you the best chance of an accurate fit number.
Show your calculations to your teacher. If they are good, you can practice size
estimation of a variety of biological specimens.