Nikon S-kt - Page 23

The working
distance is the clearance
between the upper surface of the cover
glass
and the lowest edge
of the objective when
critically focused.
Note
that,
as shown
in
Table 7,
the working distance
becomes very
small
for
high
power
objectives.
The resolution
of minimum resolved
distance
(the
limit of resolving
power)
is the
minimum distance
between object
points
discernible
as separate under the
microscope
illuminated
by light of wave length
550mpm.
The
shorter
the
wave length, the higher
the
resolving
power,
that is,
the smallest
resolved
distance. In the
table,
the
smaller values indicate the
resolution
obtained
by
oblique
and the larger
values by central illumination.
(see
"lllumination"
on
p.8)
The
minimum resolved
distance in the image is
the
value in
the
object multiplied
by
the total
magnification
of the
microscope.
lf the resolving
power
of the microscope
is important,
choose the eyepiece for
which the image resolution
falls
within that of
the naked eye
0.15-0.3mm
(when
the object is
seen
from
a distance of 25cm); the
generally
accepted criterion for
the upper limit of the total magnification of a
microscope
is
about 500-1000X of the numerical
aperture of the
objective
in use.
Note that in
photomicrography
it is
useless to
raise
the magnification beyond the
resolving
power
of
the emulsion
(usually
about
0.05mm).
However,
since
the
resolution
of the emulsion
is
higher
than that of the
naked eye,
photographs
can be
taken at
a
lower
magnification
and thereafter enlarged.
Real
field
of view
(in
mm) represents the extent of
the object that comes under
observation. For
higher magnif ication it becomes
extremely small.
Consequently, it is
advisable to
center on the object
point
to be
examined
first
under lower
magnification
and
then revolve the nosepiece to a
higher magnification.
Depth
of
focus
represents
the thickness or height of the object in
pm
sharply seen
when
observed
through the microscope.
In
photomicrography
the
depth
of focus
becomes
smaller than the figure
shown in
the
previous
table.
Therefore, careful
attention must
be
paid
to
focusing
when taking microscope
pictures.
By closing the
condenser
diaphragm, the
depth of field can
be
made larger than the
value
shown
in
the table.
When
focus
is on
the center of the field,
the
circumference will usually be blurred,
because
a curvature of
the image
plane
is unavoidable in the
microscope, except
when
using a
flat field
objective. In order to
get
sharp
edge images,
it
is
necessary to
adjust the
fine
focus knob
and shift the
focus from
the center
to the
periphery.
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