Nikon DIAPHOT 300 - Field Aperture Diaphragm

5. Field Aperture Diaphragm
The field aperture diaphragm restricts the illumination on a specimen to within an
observation area. The diameter of the field aperture diaphragm is maximized by
setting the field aperture diaphragm lever to the top position.
For general use, the diaphragm is set slightly larger (or smaller) than the viewfield.
Too wide an illuminated area gives off stray light, which causes flares, resulting in
reduced image contrast.
Therefore, correctly adjust the field aperture diaphragm, especially in
photomicrography. In general, good results are produced by stopping down the
diaphragm slightly wider than the film format. (Too much stopping down causes
vignetting.)
NOTE
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Keep the field aperture diaphragm fully opened when the SLWD
condenser is used for phase contrast microscopy.
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