IV - Microscopes - Determining Field of View

by Kathy Egbert

| 16 Questions

Review:  You are given a picture of a microorganism and told it is magnified 100 times
(written 100x). Which of the objective lens on your microscope was used to get
this magnification?

A 4x, scanning objective

B 10x, low power

C 40x, high power

The area (called Field of View) you can see through the microscope lens gets
smaller and smaller with greater magnification.

Review:  Even though you have seen this question before, it is essential that you
understand this principle when viewing specimens under the microscope. The
greatest field of view is at what magnification?

A 4x, scanning

B 10x, low power

C 40x, high power

Here is another image showing how the field of view changes with greater
magnification. Notice that the depth of the field of view also decreases greatly
with an increase in magnification.

Definition of Field of View

Because the microscope measures things that are small, we use a smaller unit of
measurement to make measurements more accurate. The unit of measurement is
called the micrometer. You need to know how to convert millimeters into
micrometers.

To increase accuracy of measuring specimens under the microscope we use the
micrometer unit of measurement.

True

False

Metric System Review

You will need to convert millimeters (mm) into micrometers

Practice: Given the measurement 3.25 mm, convert this to micrometers.

A 3250 micrometers

B 325.0 micrometers

C 32.5 micrometers

Calculating Field of View

You are now ready to begin measuring and calculating the field of view on your
microscope and completing your table. Every microscope can have a slightly
different field of view, so it will be important that you continue using the
same microscope for measuring the field of view in this worksheet.

After lining up a millimeter mark on the ruler on one side of the diameter, two
things can happen: 

1) the millimeter marks can frame both sides; just count the number of whole
millimeters (look at column 1 below)


2) the millimeter mark frames one side, but not the other (see column 2 below).
Count the whole mm marks first. Now you have to estimate the amount of area that
remains. Does it look like about 1/4, 1/3, 1/2 or ___? Add this fraction to the
whole number. You need to have the fraction in decimal format (1/2=0.5;
1/4=0.25; 1/3=0.33; 1/2=0.5; 2/3=0.66; 3/4=0.75)

Practice Measuring Diameter

Figure A measures 4 mm

True

False

Figure B measures almost exactly 5 mm

True

False

Figure C measures 1.3 mm

True

False

Table 1. Dimensions of the Field of View 

Complete Column 1 (a, b, c) by following the instructions in the above slides
and entering measured mm; 

Complete Column 2 (d, e, f) convert the mm in column 1 to micrometers by
multiplying by 1000 (this is the diameter); 

Column 3 (g, h, i) - to get the radius you divide the diameter by two, this is
"r" in the equation, use 3.14 for pi and complete the multiplication for area

Box X: complete this calculation by taking the low power field of view (Box b)
and dividing it by 4

Box Y: convert diameter in Box X to micrometers

Box Z: use same information for Column 3 above

Compare the estimated high power FOV area (Box i) with your calculated high
power FOV area (Box z). Which of the following statements best represents your
results

A Estimated area is much larger than the calculated area (i > z)

B Estimated area is about the same as calculated area (i = z)

C Estimated area is much smaller than the calculated area (i < z)

Based on your answer to Question 10, how would you rate your skills of measuring
the diameter of your FOV?

A Estimated and Calculated areas for the high power FOV were close to the same number, so I did a good job measuring the FOV with the ruler.

B Estimated and Calculated areas for the high power FOV were very different. I need to improve my measuring skills.

You now have the FOV for all three objective lens (4x, 10x, and 40x) on your
microscope because you have recorded them in Table 1, boxes g, h, and i -- Now,
let's put that information to work

Prepare a wet mount from a piece of newsprint that has some color on it. Start
with the scanning, 4x objective and bring your specimen into focus. Observe how
the colors are printed onto the wood fibers that make up the newspaper. Using
the information you've just calculated for FOV, estimate the number of colored
dots that are in one square millimeter. Choose just one of the three FOVs to do
this task. Show your work:

Another tidbit of knowledge: Like FOV (Field of View), light intensity also
decreases with higher magnification; observe how the light intensity changes in
the schematic below. This is one of the reasons that you may have to change the
amount of light coming in through the diaphragm. 

You will now use three different prepared slides from the front table to
practice your skills. Complete a Table for each slide. Specimen #1

Repeat with a different slide. Specimen #2

Repeat with another different slide. Specimen #3

One of those rare opportunities to earn some extra credit. Repeat using a
different slide - Specimen #4