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Optical Characteristics
Combinations of 10x (Field No. 18) Eyepiece with
Objectives
Objective
Total
Magnification Magnification Aperture
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
where n is the refractive index of the medium (air, immersion oil, etc.) between the
objectivelens and the specimen or condenser, and
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
is the used wavelength of light. (The resolving power in the above table is indicated
for
= 0.55 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.
Numerical
Real
Viewfield Focus
is half of the maximum angle at which
Depth of
Resolving
Power
Working
Distance
33