To estimate the actual size of microorganisms viewed through the microscope:
1. Measure the field of view diameter seen using each of the 3 objective lenses (Be sure to use the same microscope for all viewing sessions!).
a) You may measure the FOV Diameter on low power using a clear metric ruler. Record the distance in mm and µm.
b) You may measure the FOV diameter on medium and low power using special microscopic rulers. Record the distance in mm and µm.
The FOV Diameter of the medium power objective lens shown at right would be about 1.3 mm or 1300 µm.
2. Record the FOV Diameter for each of the 3 lenses in a safe place for later use. Record the diameters both in mm (millimeters) and in µm (micrometers).
3. On low power, find an object to view. You could estimate its size at this power, or center the object and switch to medium power.
4. If we use the medium power lens shown above, we know its FOV Diameter is 1.3 mm.
5. Estimate how many of the objects viewed with this lens can fit across the circular area at its widest.
6. Approximately 3.5 creatures may fit lengthwise across the middle of the field of view.
7. Divide the FOV Diameter for medium power (1.3 µm) by 3.5.
8. The organism is estimated to be 0.37 mm long, or about 370 µm.
