Leica DM R Serie Mode D'emploi page 71

Aperture diaphragm
The aperture diaphragm (48.21) determines the
lateral 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 re-
solving 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
comparison 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. 51) or Bert-
rand lens (50.2 or 54.2/54.11) 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.
objectives. Visual
Replace the eyepiece or disengage the Bertrand
lens. For objectives with low contrast the
aperture diaphragm can be stopped down fur-
ther to highlight faint specimen details. In
polarized light microscopy narrowing the
aperture diaphragm usually results in brighter
colours except for conoscopy, see page 82.
Attention: The aperture diaphragm in the
illumination light path is not for setting the image
brightness. Only the rotary brightness adjust-
ment knob or the neutral density filters should
be used for this.
An aperture diaphragm in the objective (41.3) is
normally fully opened. The reduction in image
brightness caused by stopping down results in:
Greater depth of field
Less coverglass sensitivity (p. 47)
Suitability for darkfield (p. 75)
Change in contrast
Condenser top 0.90 S 1/P 0.90 S 1
The condenser top (48.15; 16) increases the
illumination aperture, which should be about
0.6x – 1x of the aperture of the objective used.
The condenser top may therefore only be swung
out for low-power objectives. The following rule
of thumb applies for condenser tops 0.90 S 1 and
P 0.90 S1:
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