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6
Adjusting the Aperture Diaphragm
The aperture diaphragm is used to adjust the illumination
angular aperture and is important as it affects the resolution,
contrast, focal depth, and brightness of an optical image.
Turning the condenser aperture diaphragm lever changes the
size of the aperture diaphragm.
Generally, aperture settings at 70 to 80% of the numerical
aperture of the objective will provide satisfactory images with
suitable contrast.
A small aperture diaphragm opening reduces resolution and
brightness but increases contrast and depth of focus. On the
contrary, a large aperture diaphragm size increases resolution
and brightness but reduces contrast and depth of focus. These
characteristics involve inherent tradeoffs and cannot be
optimized independently.
Relationship of the aperture diaphragm size with the optical image's state
Aperture diaphragm
Stop down
Lower
Open
Higher
Proper size of the aperture diaphragm
Normally, 70 to 80% of the numerical aperture of the objective is the proper size. Since an excessively small aperture
diaphragm opening will degrade image resolution, we do not recommend setting the aperture diaphragm to less than
60% of the numerical aperture of the objective.
Adjustment timing for the aperture diaphragm
Be sure to adjust the aperture diaphragm each time you change the objective.
Adjusting the Aperture Diaphragm Using the Condenser Scale
6.1
The scale on the condenser indicates the numerical aperture.
The index on the aperture diaphragm lever should be aligned with
the scale line that corresponds to 70 to 80% of the numerical
aperture of the objective.
The numerical aperture is indicated on the side of the objective.
For a numerical aperture of 0.75, the index should be aligned with
the scale on the condenser at 0.525 to 0.6.
Adjusting the Aperture Diaphragm Using the Centering Telescope
6.2
(1)
Remove one eyepiece and attach the centering telescope in
place.
(2)
Turn the aperture diaphragm lever to stop down to the
minimum aperture. While holding down the flange of the
centering telescope, turn the eyepiece of the centering
telescope and focus on the aperture diaphragm.
(3)
Turn the aperture diaphragm lever to adjust the aperture.
Normally, the aperture diaphragm should be adjusted to
around 70 to 80% of the size of the field of view.
(4)
Remove the centering telescope and reattach the eyepiece.
Chapter 2
Individual Operations
Resolution
Brightness
Darker
Brighter
0.9
0.8
0.5
0.4
0.3
0.2
0.1
Aperture diaphragm lever
Adjusting the aperture diaphragm
Contrast
Larger
Lesser
Plan 40X
40x / 0.75
/ -WD
Indication for 40x magnification /
numerical aperture 0.75
Proper numerical aperture:
0.75 x 0.7 to 0.8 = 0.525 to 0.6
Eyepiece
Adjustment using the centering telescope
47
Focal depth
Deeper
Shallower
Chap. 2
Nikon JAPAN
Plan 40x
40x / 0.75
/ -WD
Flange
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