Chapter 4 Microscopy (Detailed Procedure)
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4
Adjusting the Aperture Diaphragm
The aperture size is increased or decreased by rotating the condenser aperture diaphragm lever.
If the aperture diaphragm is closed, the brightness and resolution are decreased but the contrast
and range of focus are increased. If the aperture diaphragm is opened, the brightness and
resolution are increased but the contrast and range of focus are decreased.
Generally, a good image of sufficient contrast can be achieved with the aperture diaphragm
closed to approximately 70% - 80% of the objective’s numerical aperture. Since the image
resolution will be degraded when the aperture diaphragm is closed too much, do not close the
aperture diaphragm to less than 60% of the objective’s numerical aperture except when
observing a specimen with low contrast, such as a near-transparent specimen.
The aperture diaphragm controls the numerical aperture of the illumination. Do not use it to
control brightness. Use the brightness control dial to control the brightness.
•
The objective’s numerical aperture is indicated on the side of the objective.
40x / 0.65 = magnifying power 40x, numerical aperture 0.65
•
To observe the diaphragm image, remove the eyepiece and look down the open
tube. (The eyepiece is fastened to the eyepiece tube. Loosen the screw before
removing the eyepiece.)
•
The figures on the condenser show the approximate positions of the aperture
diaphragm lever corresponding to each objective magnification. (When the
aperture diaphragm lever is moved to that position, the size of the aperture
diaphragm will be 70% - 80% of the objective’s numerical aperture.) Each
time you switch the objective, align the aperture diaphragm lever to the same
figure as the magnifying power of the objective to get a good image with
sufficient contrast.
Corresponding objective magnification
Aperture diaphragm
lever
Magnifying Power: 40x
Numerical Aperture: 0.65
Objective pupil
Aperture diaphragm image
The appropriate size of the aperture diaphragm
Summary of Contents for eclipse E200
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