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Optical Characteristics
Combinations of 10x (Field No. 20) Eyepiece
1
with E-Plan Objectives
Objective
Total
Magnification
Magnification
4×
10×
40×
100×
Microscope Terminology
2
(1) Total Magnification
The total magnification of a microscope is the individual magnifying power of the
objective multiplied by that of the eyepiece.
(2) Numerical aperture (N.A.)
The numerical aperture is an important factor in determining the efficiency of the
condenser and objective. It is represented by the formula:
N.A. = n sin α
where n is the refractive index of the medium (air, immersion oil, etc.) between the
objectivelens and the specimen or condenser, and α is half of the maximum angle at
which light enters or leaves the lens from or to a focused object point on the optical
axis.
The larger the numerical aperture the brighter the image and the higher the resolution.
(3) Resolving Power
The ability of an optical system to discriminate between two discrete objects separated
by a minute distance. The more minute the distance, the higher the resolving power of
the optical system. In relation to the numerical aperture, the resolving power is
represented by the following formula:
Resolving power =
where λ is the used wavelength of light. (The resolving power in the above table is
indicated for λ= 0.55µm.)
(4) Working Distance (W.D.)
The clearance between the front of the objective and the upper surface of the
coverglass, when a specimen image is sharply focused. Generally, the higher the
magnifying power of the objective, the shorter the working distance.
Numerical
Real
Aperture
Viewfield
40×
0.1
100×
0.25
400×
0.65
1000×
1.25
λ
×
Ν
.Α
.
2
75
Depth of
Resolving
Focus
Power
5 mm
63.2 µm
10.1 µm
2 mm
0.5 mm
1.2 µm
0.2 mm
0.4 µm
Working
Distance
2.8 µm
30 mm
1.1 µm
7 mm
0.4 µm
0.65 mm
0.2 µm
0.23 mm
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