Olympus CX33 Instructions Manual page 26

Aperture diaphragm
The aperture diaphragm is a diaphragm to adjust the numerical aperture of the condenser.
Adjusting the numerical aperture of the condenser appropriately with respect to the numerical aperture of each objective
allows you to observe the specimen with the best contrast and resolution.
In general, since the contrast of the specimen to be observed
with microscope is low, it is appropriate to adjust the
numerical aperture of the condenser to approximately 70 to
80% of the numerical aperture of the objective.
The rough indication is that the aperture diaphragm image
viewed when looking into the tube without eyepieces looks as
the picture shown on the right.
Objective
Plan objective
22
Numerical aperture of objective and numerical aperture of condenser suitable for observation
Magnification
4X
10X
20X
40X
100XO
30-20%
Numerical aperture of
objective
NA
0.10
0.25
0.40
0.65
1.25
70-80%
Aperture diaphragm image Aperture diaphragm image Aperture diaphragm image
Pupil of objective Pupil of objective
(bright circular ring)
Numerical aperture of condenser suitable for
observation of specimen for microscope
(when setting to 70% of objective)
0.10 x 0.7 = 0.07
0.25 x 0.7 = 0.175
0.40 x 0.7 = 0.28
0.65 x 0.7 = 0.455
1.25 x 0.7 = 0.875