Microscopy
Formulas
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Magnification
Magnification =
Magnification of
Eyepiece lens
X
Magnification of
Objective lens
If the eyepiece has a magnification of 10X, and the objective
lenses are 4X, 10X, and 40X, what are the magnifications at
low, medium, and high power?
Low Power
= 10 x 4 = 40X
Medium Power = 10 x 10 = 100X
High Power
= 40 x 10 = 400X
Example:
Field of View
at Low
Power
Computing
FOV of other
objectives
If the diameter of field of
view (FOV) is KNOWN for
low power, you can
compute for the FOV for
the medium and high
power
FOV(low) x magnification (low) = FOV(med) x magnification (med)
5 mm x 40X = FOV(med) x 100X
FOV(med) = (5 mm x 40X ) / 100X
FOV(med) = 200 / 100 = 0.2 mm
You often
need to
compute
FOV into µm
Magnification
FOV (mm)
FOV (µm)
Low power
(4X)
40X
5
5,000
Medium
power (10X)
100X
2
2000
High power
(40X)
400X
0.5
500
• 1 mm = 1,000 µm
Computing Object Size (1)
1. Estimate how many times the specimen fits
across the FOV
2. Example on the left: specimen fits across the
high power FOV approx. 2.25 times
times the object
3. Object size = FOV (in µm) / #fitsofacross
FOV
4. Object size = 500 µm / 2.25 = 220 µm
Computing Object Size (2)
Example:
• The average size of an onion skin cell
was given at 0.25 mm
• Magnification = Image size
Actual size
• The image printed on a worksheet
measures 2 cm
• What is the magnification of the
image?
= 20 mm/ 0.25 mm
= 80 mm
•
= 80,000 µm
Compute for Drawing Magnification
Drawing magnification = Drawing size / Object Size
Example:
Drawing of an onion skin cell was measured at 4 cm
Actual size of an onion skin cell was given at 0.25 mm
Drawing magnification
= 40 mm / 0.25 mm
= (40,000 µm / 250 µm)
= 160 times