For low-contrast objects, the aperture dia-
phragm can be closed further to highlight even
faint specimen details. In polarised light
microscopy, narrowing the aperture diaphragm
usually results in brighter colours.
Attention:
The aperture diaphragm in the illumination
beam path is not for setting the image
brightness. Only the rotary brightness adjust-
ment knob or the neutral density filter should be
used for this purpose.
Fig. 46 Transmitted-light illumination unit with condenser
1 Markings, 2 Catch lever for condenser adjustment, 3 Aperture
diaphragm
3
1
1
44
An aperture diaphragm in the objective is usually
fully opened. The reduction in image brightness
caused by stopping down results in:
Better depth of field
Lower coverglass sensitivity
Suitability for darkfield
Change in contrast
Possible errors
Wrong coverglass thickness or wrong objective.
Specimen has been placed on the stage with the
coverglass upwards instead of downwards.
Aperture diaphragm opened too wide or closed.
Condenser at incorrect level.
Light ring inadvertently used.
IMC component inadvertently used.
Dirty optics.
2