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Objective, Total Magnification and Useful Magnification
47
Adjusting the Con-
denser
Now the illuminated area is optimally set.
1)
Open the field diaphragm so that its edge is still completely within the
visible area.
2)
Center the image of the field diaphragm using the condenser's adjust-
ment screws.
"
A precisely adjusted condenser guarantees that only the area of the
specimen to be viewed is illuminated.
3)
Repeat steps 1) and 2), until the edge of the field diaphragm is no long-
er visible in the visual field.
Adjusting the Lamp
You can generally skip adjusting the lamp with modern microscopes, provid-
ed that the lamp is located in a frame and the bulb is in a centered socket.
Objective, Total Magnification and Useful Magnification
There are no general rules for which objective you select; one should always
keep the specimen and reason for observation in mind.
Terminology
The two most important parameters of an objective (magnification and nu-
merical aperture) are engraved in the objective. First the magnification and
then the numerical aperture which is separated by a diagonal slash. Hence,
the code 40 / 0,65 marks an objective with a magnification of 40X and the
numerical aperture of 0,65.
Magnification
An objective's engraved magnification is considerably important for the total
magnification. However, one should note that the range of useful magnifica-
tion should not be surpassed.
Numerical Aperture
The numerical aperture is the characteristic of an objective which deter-
mines its capacity. The numerical aperture can also be found engraved into
the objective. It is based on the refraction index of the lenses being used as
well as the objective's angular aperture. The manufacturers are narrowly
limited when dealing with the refraction index, whereas the objective's angu-
lar aperture can be increased by using appropriate measures. To achieve
this, a fluid with a high refraction index is placed between the cover slip and
the objective. The immersion objectives are accordingly marked and should
only be used with the appropriate immersion fluid.
Total magnification
The system's total magnification, to which an installed TV adaptor contrib-
utes, is based on the actual size of the final image. This can be the image
on the monitor, as well as the image printed on paper. To determine the total
magnification, it is best to use a stage micrometer which can be shown on
the monitor or the printout. You can determine the total magnification by di-
viding the length of the object shown by the set length on the micrometer.
Useful Magnification
The term useful magnification specifies a total magnification of the optical
system by which full resolution is used without having a negative effect on
contrast or sharpness. The useful magnification of the optical systems mi-
croscope depends on the numerical aperture being used ( engraved on the
objective). The useful magnification is in the range of 500x to 1,000x the nu-
merical aperture.
Empty Magnification
A loss of contrast and sharpness is the result if total magnification is larger
than the useful magnification. Check to see if replacing the TV adapter with
a 0,5 or 0,63 model sufficiently reduces the total magnification. If this is not
possible, you can also use an objective with the same magnification, but
with a higher numerical aperture.
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