as small-detail reproduction is concerned, not
of course as far as large sized detail and macro
detail and tone-reproduction are concerned.
The density compression shall take place in
all areas which carry high light and shadow
details of interest, and for that reason the un-
sharpness of the mask for the scanning spot
shall be small, in the order of the detail itself,
say one-tenth to one millimetre. An important
point to note is that existing dodging printers
do not yet fulfil these requirements, and are
consequently not yet of full value to photogram-
metry as far as economy of micro details is
concerned. We hope that it is possible for im-
provements to be made in the near future.
In cases where the photo scale is determined
by the recognisability of small detail, this has
a great bearing on the survey economy, because
the photoscale is determined then by the repro-
duction of detail. The necessity of adapting the
photo scale can account for a factor of two in
the photo scale; consequently in a factor two
even in the number of photographs. This work
has shown that these conclusions are only valid
for very small detail exclusively. For small
objects the resolution criterion is basically
wrong, and we must use there a contrast trans-
mission if we want to judge performance. The
positive process, including dodging, can only be
used by contrast transmission as well as judging
the contents of photographs and judging inter-
pretation of aerial photographs.
Mr A. J. WATSON: I would like to say that in
my field, which is the fluorododge printing field,
we are researching towards the fineness as
mentioned by the previous speaker, and towards
the overall compensation which will approach
nearly those limits by principally projection
methods rather than by contact methods, which
in turn will give greater resolution results. We
are continuing such development at the present
moment.
Mr G. C. Bnock: Referring to Mr Corten's
paper which he has just summarised, I am not
sure whether Mr Corten said at the platform
what he said in the paper, which seems to imply
that the paper print can be superior to the dia-
positive. I think it should be quite clearly un-
derstood that we have to specify the character-
istics of the diapositive and that if you have a
diapositive of fine enough material or grain you
will certainly be able to do as well as you can
on a paper print. In other words, I would sug-
gest that the experiments done at I T C do not
extend to a positive transparency material of
104 PROGRES ACCOMPLIS, DISCUSSION
fine enough grain and high enough resolving
power.
I would expect from the shapes of the fre-
quency response curves of negative and positive
materials that if you want to reproduce cor-
rectly all the detail in a given negative, you have
to use a positive material whose resolving power
is at least three times that of the negative emul-
sion — at least three times, perhaps more — and
commonly available diapositive materials are
nothing like as good as that. However, they can
be obtained and I have seen plenty of evidence
that given the right kind of positive material all
the detail in a good negative can be evaluated.
I think it is merely a matter of choosing the con-
stants directly, it is not a fundamental disagree-
ment with Mr Corten’s paper.
Mr F. L. CorTEN: We have indeed not used
various positive materials, but we used the best
positive material which is commonly available
and commonly used in aerial survey, but I did
not know there were better transparencies
available.
It was not the intention, by the way, to state
that the performance of a paper print can be
better than a diapositive transparency. How-
ever, I would like to make another remark: it
is dangerous to confuse discussion about reso-
lution and performance, that is to confuse dis-
cussion about resolution and about contrast
transmission. It is a very dangerous thing. I
think it would be much better in future if
when we speak about resolution we just mean
that minute micro detail only, and if we want
to judge image performance as it interests the
photogrammetrist — that is details in the pho-
tograph which are of the order of size of
a tenth of a millimetre or one millimetre —
we do not speak of resolution any more as
a criterion, but that it must basically be judg-
ed by something like contrast transmission
where the frequency does play a role. 1 know
we have discussed that and we agreed on this
point, but I wanted to state this in the audito-
rium. One of the conclusions of this paper -
one of my conclusions, as I told you — was that
we started using the resolution as a criterion but
we found that particularly in judging positive
processes, scanning printers and so on, this cri-
terion was basically wrong and should not be
used, but that exclusively contrast transmission
should be used.
As far as the remark about Kel-O-Wat is
concerned I am in complete agreement. We also
measured Kel-O-Wat contrast transmission of
the Kel-O-Wat printer, and we came to the con-
clu:
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