Olympus EHT-431 Buku Petunjuk - Halaman 18

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Halaman Cetak

60-10"

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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.

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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