Leica DMLS, Instructions Manual

Page: 32 / 160

32
Open the field diaphragm (23.10) until it just
disappears from the field of view (24d). When
changing an objective the condenser centration
may have to be slightly adjusted.
The field diaphragm (23.10) protects the speci-
men from unnecessary warming and keeps all
light not required for image formation away from
the object to enable greater contrast. It is
therefore only opened just wide enough to
illuminate the viewed or photographed object
field. A change in magnification therefore
always necessitates matching of the field dia-
phragm
→
light path p. 6.
Aperture diaphragm
The aperture diaphragm (23.7) determines the
resolution, depth of field and contrast of the
microscope image. The best resolution is
obtained when the apertures of the objective
and the condenser are roughly the same.
When the aperture diaphragm is stopped down
to be smaller than the objective aperture,
resolving power is reduced, but the contrast is
enhanced. A noticeable reduction in the
resolving power is observed when the aperture
diaphragm is stopped down to less than 0.6x of
the objective aperture and should be avoided
where possible.
The aperture diaphragm is set according to the
viewer’s subjective impression of the image, the
scale on the dial is just to allow reproducible
settings and does not represent absolute
aperture values. In principle you can do a
calibration yourself by comparison with the
apertures of various objectives. Visual com-
parison of the apertures of the objective and the
condenser can be made as follows: Remove the
eyepiece from the eyepiece tube or engage an
auxiliary telescope (Fig. 25.1) (
→
p. 35) and
focus. Close or open the aperture diaphragm
until its image is just visible in the objective pupil
(brighter circle). This is considered the standard
setting, i. e. condenser aperture = objective
aperture.
For objectives with low contrast the aperture
diaphragm can be stopped down further to
highlight faint specimen details. In polarized
light microscopy narrowing the aperture dia-
phragm usually results in brighter colours.
n.b.:
The aperture diaphragm in the illumination light
path is not for setting the image brightness. Only
the rotary brightness adjustment knob or the
neutral density filters should be used for this.