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23
N. OPERATIONS IN DETAIL
9. Use of Condenser Aperture Diaphragm
(Condenser aperture diaphragm control ring)
1) For orthoscopic microscopy
The condenser aperture diaphragm IS provided for adjusting the
numerical aperture (N. A.) of the illuminating system of the microscope.
It IS important because It determines the resolution of the image,
contrast, depth of focus, and brightness.
Stopping down the aperture diaphragm will lower the resolution and
brightness but Increase the contrast and depth of focus.
In general,
when it is stopped down to
70%~80%
of the numerical aperture of the
objective, a good image of appropriate contrast will be obtained
(Fig.l 6).
Exit pupil of
objective
Aperture
diaphragm
Size of the condenser aperture diaphragm
Fig.16
To adjust the size of the condenser aperture diaphragm, manipulate the
diaphragm control ring referring to the condenser's N. A. scale, or
observing the diaphragm Image VISible on the exit pupil inside the
objective. The eXit pupil can be seen by removing the eyepiece from the
eyepiece tube or putting the Bertrand lens into the optical path.
Stopping down the aperture diaphragm too far will lower the resolVing
power. Therefore, it IS recommended not to stop down the aperture to
a size smaller than 60% of the N. A. of the objective in use, except when
observing almost transparent specimens.
I
Note
I
When SWinging out the top lens of the condenser (for
microscopy using 4
x
or lower objective), fully open the
condenser aperture diaphragm.
2) For conoscopic microscopy
In conoscopic microscopy, the condenser aperture diaphragm works as
a field diaphragm on the conoscopic Image surface. Stop down the
diaphragm to such an extent that it clrcumscnbes the Circumference of
the field of view of the conoscopic Image (exit pupil of objective) to shut
out the stray light.
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