Leica Microsystems DM2000 Manuel D'utilisation page 132

8. Operation
8.10 Field Diaphragm
The field diaphragm (58.2, 59.5) protects the
specimen 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 illu-
minate the viewed or photographed object field.
A change in magnification therefore always ne-
cessitates matching of the field diaphragm.
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