12 THE PHOTOGRAPHIC IMAGE, BROCK
development conditions often indicate a gamma much higher than unity, which must give
an enhancement of object contrast in the image before the response begins to droop with
rising frequency. It would always be desirable to state whether the CT curves have been
converted to an arbitrary unity at low frequencies and to define the sensitometric con-
ditions as completely as possible.
24.2. Understanding of image phenomena.
Some curves showing the performance of lenses and emulsions as a function of fre-
quency are given in Figs. 6 to 10. While only a limited sample these will serve as an
o
LOG CT.
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|
-20 H1)—153 mm. f/5-6 PHOTOGRAMMETRIC LENS -0°— |
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| | Ee
|
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©) 153mm f/2 TELEVISION LENS-75°
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@ so mm. t/2 LENS lor | |
d a cxx ll
[9] 1.0 2.0
LOG (LINES/mm.)
Fig.6. CT curves for three lenses.
introduction and can be used to illustrate a number of points about resolving power and
allied facts of photographic imagery which were formerly hard to explain.
In Fig. 6 are shown CT. curves for three lenses of different type, representing
various standards of image quality. An f/5.6 lens of perfect correction would have unity
C.T. out to say 100 lines/mm. At zero frequency the curves all pass through 0 on the “log
C.T." scale since a unit contrast (black and white) test-object is in use. Had the test-
object been of the familiar low-contrast (0.2), all the curves would have been displaced
vertically downwards by —0.66. A horizontal line drawn at the contrast level of 0.048
(a somewhat arbitrarily assumed figure for contrast-sensitivity of the eye) would inter-
sect each lens curve at the frequency of its visual resolving-power. Bearing in mind that
photographic resolving-powers (black and white test object) lie in the region 50-100
lines/mm, it is interesting to note that although lens (3) would have much higher visual
resolving-power (in agreement with its larger aperture), this advantage is not main-
tained in the photographic region, where it is substantially equal to the smaller aperture,
longer focal-length, photogrammetric lens. Lens (2), tested near the limit of its field, is
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