Nikon ECLIPSE Ei, Instructions Manual

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Chapter 3 Detailed Explanation
59
3.9 Optical Characteristics

Combinations of 10x (Field No. 20) eyepiece with BE2-Plan objectives
Objective
Magnification
[x]
Total
Magnification
[x]
Numerical
Aperture
Real Viewfield
[mm]
Depth of
Focus
[
μ
m]
Resolving
Power
[
μ
m]
Working
Distance
[mm]
4 40 0.1 5.0 115.0 2.8 25
10 100 0.25 2.0 18.4 1.1 6.7
20 200 0.4 1.0 6.1 0.7 3.7
40 400 0.65 0.5 2.0 0.4 0.6
60 600 0.8 0.3 1.2 0.3 0.25
100 1000 1.25 0.2 0.7 0.2 0.14
3.10 Microscope Terminology
(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 objective and the
specimen or condenser, and
α is 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 =
. . 2 Α Ν ×
λ
where λ is the used wavelength of light.
(The resolving power in the above table is indicated for λ = 0.55 μ m.)
(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.
(5) Field Number of the Eyepiece
The diameter in mm of the viewfield observable through the eyepiece. When an eyepiece has an
indication of “10x / 20”, it means that the magnification is 10x and the field number is 20 for that eyepiece.
(6) Real Viewfield
The diameter of the region of the specimen that is actually observed with the microscope.
Real viewfield = field number of eyepiece / magnification of objective
(7) Depth of Focus
The depth (thickness) of the specimen image in focus, extending above and below the focused image
plane. The larger the N.A. of the objective, the shallower the depth of focus.
DOF
:
n :
ω
:
M :
N.A
. :
λ :
DOF = n ×
M
×
N.A.
250000
×
ω
+
2
×
N.A.
2
λ
Depth of focus (object side)
Refractive index (Varies depending on the medium between the specimen and the objective lens)
For air: 1
For oil immersion (Nikon Immersion Oil): 1.518
Eye resolution (5’=0.0014)
Total magnification
Numerical aperture of the objectives
Wavelength (observation magnification: 0.55
μ m.)