sorption, scatter.
ition, absorption,
vister glass
mmetry.)
ion, exposure.
n.
cess. The ideas of
ell known to phy-
to the majority of
topical interest to
ical treatment by
re may be accept-
anding importance
, Other than those
tic system can be
), size, and bound-
's in areas of dif-
yunded by definite
tin a photograph,
close resemblance
e general imagery,
by the lines of a
a rapid change of
on stimulated this
THE PHOTOGRAPHIC IMAGE, BROCK 7
luminance, but it is curious that to the scanning mechanism of the eye and brain the
negative step-impulses of luminance in Fig.1 can stand for the complex changes of
luminance in the (imagined) original scene.
The total information capacity of a photo- a
graphic system must be related to the num- CS
ber of steps of tone and to the number of Ca
boundaries which it can record. This implies c e
contrast reproduced down to the smallest 9
possible size. 9 9
v J
2.2. Approaches to assessing the photo-
graphic image. (
The act of seeing, conceived as a scan-
ning process, has connotations of changing 0% —
signal intensity on passing over an edge, l| ^
and hence of rate of change as a measure
of *sharpness". Likewise, as the eye sweeps Fig.1. Information conveyed by symbol-
across line-grids of progressively finer spa- ised boundaries. (Reproduced by permis-
cing there is an obvious connection between sion of PUNCH.)
the temporal signal variation and the spatial
line separation. This would not have been so readily appreciated when photographic
resolution testing was first being worked out some twenty-five years ago, vision being
then for most people a statie concept. Without the basie concept of frequency, the need
for measuring the efficiency of the system as a continuous function of detail size would
not have appeared very urgent. Certainly the photographie engineers of the time (or
most of them) concentrated their thoughts on the limit of what their system could do,
the resolving-power, and ignored the territory between this and the macro-region, where
the orthodox tone-reproduction diagrams could be applied. This separation between the
macro-region and the resolution limit was accepted for a long time, with different
people studying the two regions, yet the unworked territory between holds equal interest
for air photography. Looking back, it seems strange that this situation persisted so long,
but explanations can be found. Apart from the natural tendency to persevere with known
ideas, the most probable reason is simply the instrumental difficulty of measuring any-
thing but resolving power. It is fairly easy to set up a collimator and graticules for a
resolution test visual or photographie, and assessment of the results for “resolved or not”
requires little beyond human eyes and brains, which are commonly available instruments.
But to measure response in the intermediate size zone requires not only a new mental
approach but additional instruments, for the unaided eye will not provide scale readings,
good though it is as a null detector. These further instruments have to be of quite con-
siderable precision and require greater (or at least different) skill in use.
2.3. The television case.
Television engineers have approached their analogous problems in a more flexible
way which repays consideration. They were fortunate in that they thought naturally
about their amplifiers and landlines in terms of a response which was sensibly flat over
the working range of frequency and dwindled beyond it. A working limit would be the
frequency where the response had declined slightly, say to one half of its level value,
rather than the condition of zero response which would correspond to the photographic
resolution limit. Signal vanishing into noise is perhaps a closer analogy; the corre-
spondences are not exact, but the essential point is that the photographer could only
specifiy the size at which his equipment showed no image modulation, while the elec-
tronic engineer was well used to measuring his response as a continuous function down-
