Magnification and Size
Measuring biological specimens
using millimetres as units
SBI3U
Body Tube
Ocular lens
(Eyepiece)
Nosepiece
Objectives
Arm
Stage
Stage Clips
Diaphragm
Light
Coarse
Adjustment
Fine Adjustment
Base
Always carry a microscope with one hand
holding the arm and one hand under the
What’s my power?
To calculate the power of magnification, multiply the power of
the ocular lens by the power of the objective.
What are the powers of
magnification for each of
the objectives we have on
our microscopes?
Field of View (FOV)
Field of View – the view you actually see
when you look through a microscope
Low Field of View (Low FOV)
Low FOV
= measured with a ruler on the
microscope
• Count the spaces you see
• Count the halves as well
• Here we see 5.5 spaces
• So Low FOV = 5.5 mm
Medium & High Field of View (FOV)
Medium FOV
= low FOV
High FOV
= low FOV
X _MagnificationLOW
MagnificationMEDIUM
X _MagnificationLOW
MagnificationHIGH
REMEMBER…we only use a ruler for LOW FOV
Let’s Practice
If the low FOV = 2mm, low magnification is
4x, medium magnification is 10x and high
magnification is 40x, calculate the high
FOV.
Let’s Practice
If the low FOV = 2mm, low magnification is 4x,
medium magnification is 10x and high
magnification is 40x, calculate the high FOV.
High FOV = 2mm x _____4x___
40x
= 2 mm x 0.1
= 0.2 mm or 200µm
Let’s Practice
If the low FOV = 2mm, low magnification is 4x,
medium magnification is 10x and high
magnification is 40x, calculate the high FOV.
High FOV = 2mm x _____4x___
40x
= 2 mm x 0.1
= 0.2 mm or 200µm
Specimen Size
You can estimate specimen size using the field
of view. For example, if your field of view is
200 µm, and the specimen takes up about a
quarter of the space, you can estimate it to be
about 50 µm.
Drawing Magnification/Scale
• When you are asked to draw what you see
under the microscope, your drawings will be
much larger than your specimen. You need to
indicate approximately how much larger than
life your drawings are.
Magnification = drawing size
Actual size
or
M= D
A
Drawing Magnification/Scale
• e.g. a student, observing an amoeba under a
magnification of 400X, calculates that it is
about 50m long. If she then draws the
amoeba 8 cm long, what is the magnification
and scale of her drawing?
Magnification = drawing size = 80000 µm
Actual size
50 µm
= 1600 X, and the scale is 1cm = 6.25 µm.
To accurately measure the size of cellular
structures we need a suitable scale:
Ideally, we need a scale we can see directly
alongside the cells we are observing:
Start by putting a ruler under the microscope:
Appearance of ruler at medium magnification
Appearance of tissue at medium magnification
Estimating cell size at medium magnification
12
1 mm
1mm / 12 cells =
0.083 mm
5
1mm / 5cells=
0.2 mm
1 mm
Other units
Diameter of field
of view/mm
No. of cells
lengthways
No. of cells
widthways
Mean length (mm)
Mean width (mm)
1.00
5
12
0.2
0.083
1mm = 1000µm
Mean length of cells = 0.2 x 1000 = 200µm
Mean width of cells = 0.083 x 1000 = 83µm
Calculating actual size:
Calculating magnification & actual size:
Calculating actual size:
Calculating magnification:
Calculating magnification & actual size:
Calculating actual size:
Drawing Magnification/Scale
 e.g. a student, observing an amoeba under a
magnification of 400X, calculates that it is
about 50m long. If she then draws the
amoeba 8 cm long, what is the magnification
and scale of her drawing?
Magnification = drawing size = 80000 µm
Actual size
50 µm
= 1600 X, and the scale is 1cm = 6.25 µm.