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Use of Iris Diapnragms
A field iris diaphragm as well as an aperture
iris diaphragm is provided on the microscope.
The field iris diaphragm is built into the
base and the aperture iris diaphragm is part
of the condenser.
1.
Field Iris Diaphragm
The field iris diaphragm controls the diam-
eter of the ray bundle impinging on the
specimen surface and thus increases image
definition. Stop down the field iris dia-
phragm while looking through the eyepiece.
An image of the iris diaphragm will appear
within the field. Now open the field dia-
phragm until its diameter is just slightly
larger than the diameter of the field of view.
• When particularly clearer definition of
an image is required in the center of the
field of view stop down the iris diaphragm as
narrow as shown in the picture at bottom.
• The image of the field iris diaphragm is conjugated on the specimen's
surface, so that the diameter of the field iris diaphragm changes
according to the change of the objective power. By the same token
with every change of the eyepiece the field number will be varied,
which necessitates re-adjustment of the diameter at the field diaphragm.
2. Aperture Iris Diaphragm
An aperture iris diaphragm opened too
wide impairs image con·trast due to internal
reflections and related factors. On the
other hand, if the diaphragm is stopped
down exce"->5ivcly, rC50lution is unduly
reduced. It is therefore suggested to match
the opening of the aperture iris diaphragm
to the numerical aperture of the objective
in use, in order to achieve maximum
objective performance. For that purpose
simply set the numerical aperture scale on
the condenser to the numerical apeflure of
the objective in use.
However, since microscopic specimens generally are low in contrast,
their image lacks contrast if the objective is usecl with its full numerical
aperture. Therefore. il is occasionally preferable to stop down the aperture
iris diaphragm slightly more than indicated by the objective N.A. This
will result in increased image contrast, larger depth of focus and a flatter
field. On the other hand, stopping down too much impairs resolution.
An aperture setting of
O.6-0.7x
the N.A. of the objective is recommended.
If the N.A. of the objective is 1, for instance, you can set the scale to
0.6-0.7.
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