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2. Aperture Diaphragm
Aperture diaphragm dial
Stop down the aperture diaphragm to 70~80% of the
numerical aperture of the objective in the optical path.
Remove one of the binocular eyepieces, and look into
the eyepiece sleeve. The objective's exit pupil is visible
there. As you tum the aperture diaphragm dial, the
aperture diaphragm image becomes visible on that exit
pupil. Adjust the size of that image to the 70~80%
range of the exit pupil to obtain good image.
*
The aperture diaphragm is used to adjust the numerical
aperture (N.A.) of the illumination system, and plays an
important role in determining image resolution, contrast and
depth of focus. Since resolution is reduced when the aperture
diaphragm is stopped down more than necessary, do not stop
it down to 60% or below of the N.A. of the objective in the
optical path except when a special sample is observed.
*
The aperture diaphragm image may not be visible when an
uneven or a low reflectance sample is in the optical path.
Replace the sample with a flat one having high reflectance,
and tum the aperture diaphragm dial to adjust the size of the
aperture diaphragm image as described above.
35
Objective's
exit pupil
Image of
the aperture
diaphragm
The size of aperture diaphragm
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