62
1.9 Parfocality
The depth of field (axial resolution) depends on
the objective aperture and the magnification; it
is under 1
µ
m for highest magnification
objectives.
In principle, it is possible to achieve absolutely
perfect parfocality (identical focusing) of all
objectives used on the nosepiece by
mechanical and optical means, but this is
extremely complicated. It would be noticeably
impaired even by the torque and any dust
particles on the objective shoulders when the
objectives were screwed in. All the same, the
parfocality on Leica microscopes with
mechanical focusing is so precise that only
slight refocusing is necessary after each
objective change. Using the motor focus, this
parfocality can even be perfected with
automatic focus correction through the motor
focus and coded nosepiece for each objective
after one calibration.
Attention:
Please read the following imp ortant information
before storing the objective focus offsets:
Screw all objectives into the nosepiece with
about the same torque. If the nosepiece is
interchangeable, make sure it fits properly in the
microscope and keep the contacts clean. The
eyelenses of the eyepieces must be exactly
focused on the intermediate image. This is only
possible by inserting a (random) graticule in the
eyepiece or the Vario tube.
Another suitable focus indicator for the eye-
piece eyelenses is any overlay of a photomicro
device or the MPV microscope photometer.
However, it is not sufficient to focus on the edge
of the eyepiece field diaphragm or on diapositive
overlays (see page 101).
Attention:
When the viewer changes his glasses or when a
different person looks through the microscope
the focusing of the eyelens(es) should always
be checked and corrected if necessary. When
the eyelens is not properly focused the focal
plane of the objectives varies by different
amounts, which can cause focusing errors and
even collisions between specimen and
objective.
Adapted TV cameras may have a different focal
plane compared with that for direct observation.
This may be caused by tolerances in the flange
focal length of the objective of the camera; the
flange focal length can be adjusted for some TV
cameras.
Objectives with coverglass information “0” must
not be used for covered specimens; only use
objectives with the engraving “–” (i.e. for use
with or without a coverglass, see page 48) and
“0.17” (only with 0.170 mm coverglass). For
heating stages with an observation window, H
PLAN heating stage objectives with engraving
1.8 Q (i.e. for 1.8 mm quartz glass window) and
“–” objectives can be combined.
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