9.12.
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321
print exposure time of more than a minute is required, some fogging is
almost certain to occur which will reduce contrast and print quality.
Furthermore, in attempting to enlarge dense negatives, the lens diaphragm of the enlarger is wide open thus reducing its depth of focus,
probably to the point where all parts of the image are not in exact focus.
In cases of extreme overexposure it is best to abandon the negative for it
will not reduce uniformly, and in all likelihood prints will be mottled.
In spite of the fact that some reducers are called proportional, the
net effect is to heighten contrast in a negative that has been reduced.
At times this may be an advantage, but it can also be a disadvantage if
excessive. Thus, under most circumstances, reduction should be minimal.
Underexposed negatives are a more serious problem than overexposed
ones. In general, if one cannot get a satisfactory print by straightforward
enlargement on the hardest enlarging paper, one is best off discarding
the negative. Attempts to salvage it are apt to be frustrating. The use of
standard intensifiers is possible when only a small degree of intensification is necessary. They will not boost decidedly underexposed negatives
to useful levels.
High contrast developers can be used with enlarging papers which
help somewhat in improving contrast. Most packaged developers can be
used without dilution, in a solution two or three times as strong as is
normally recommended and, under these circumstances, will yield greater
print contrast. "Antifog" agents can be used with developers which will
heighten contrast a little, but produce unfortunate bluish tones. Such
prints can be used for publication, however.
Contrast can be greatly increased by making a contact positive print
of the underexposed negative on a lantern slide plate. Then yet another
contact print, a second negative, is made from this. In each step contrast
will be increased. One pays for this, however, in excessive grain, and the
final negative is not apt to be useful if much enlargement is required.
9.12. Interpretation of Electron Microscope Images
It is easy not to realize or one forgets that electron images differ from
photon images in very important respects. In a fundamental sense an
electron micrograph is more closely allied to an X-ray picture than to
a photograph. It is a shadowgram in which one records mass-density
differences. Although one cannot ordinarily quantitate the tonal varia-
INTERPRETATION OF ELECTRON IMAGES
321
print exposure time of more than a minute is required, some fogging is
almost certain to occur which will reduce contrast and print quality.
Furthermore, in attempting to enlarge dense negatives, the lens diaphragm of the enlarger is wide open thus reducing its depth of focus,
probably to the point where all parts of the image are not in exact focus.
In cases of extreme overexposure it is best to abandon the negative for it
will not reduce uniformly, and in all likelihood prints will be mottled.
In spite of the fact that some reducers are called proportional, the
net effect is to heighten contrast in a negative that has been reduced.
At times this may be an advantage, but it can also be a disadvantage if
excessive. Thus, under most circumstances, reduction should be minimal.
Underexposed negatives are a more serious problem than overexposed
ones. In general, if one cannot get a satisfactory print by straightforward
enlargement on the hardest enlarging paper, one is best off discarding
the negative. Attempts to salvage it are apt to be frustrating. The use of
standard intensifiers is possible when only a small degree of intensification is necessary. They will not boost decidedly underexposed negatives
to useful levels.
High contrast developers can be used with enlarging papers which
help somewhat in improving contrast. Most packaged developers can be
used without dilution, in a solution two or three times as strong as is
normally recommended and, under these circumstances, will yield greater
print contrast. "Antifog" agents can be used with developers which will
heighten contrast a little, but produce unfortunate bluish tones. Such
prints can be used for publication, however.
Contrast can be greatly increased by making a contact positive print
of the underexposed negative on a lantern slide plate. Then yet another
contact print, a second negative, is made from this. In each step contrast
will be increased. One pays for this, however, in excessive grain, and the
final negative is not apt to be useful if much enlargement is required.
9.12. Interpretation of Electron Microscope Images
It is easy not to realize or one forgets that electron images differ from
photon images in very important respects. In a fundamental sense an
electron micrograph is more closely allied to an X-ray picture than to
a photograph. It is a shadowgram in which one records mass-density
differences. Although one cannot ordinarily quantitate the tonal varia-