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GEOMETRISCHE EIGENSCHAFTEN DES BILDES, DISCUSSION 57
is not intended to restrict improvements or ne-
cessarily changes in calibration techniques. It is
always important that, with a standard, it
should not hinder progress. However, I would
not like this to be used as an excuse to depart
from the specification where it was not really
necessary, and I think it is most important that
we should make use of this recommendation as
widely as possible in order that different labo-
ratories will agree with one another.
Also in the preamble, a paragraph has been
added to point out the importance of providing
complete identification data for cameras. We
have emphasised the desirability of making
resolution tests with the same sensitive materials
as have been used in actual practice, and a sta-
tement of the maximum resolving power of the
photographic material has been required. Some
references are given to methods of obtaining
such data, we have reduced the significance at-
tached to average resolving power and empha-
sised the need for complete resolution data over
the field.
A note has been added on the differences
which occured between laboratory calibration
and the service use of a camera. We have defined
average radio-measured distortion as a step to-
wards defining a principal point of best sym-
metry. Although the principal point of best
symmetry is now defined in the recommenda-
tion as a new term, it should be understood that
the principal point of autocollimation and the
distortion relevant to it remain the basic items
which must be reported. Additional data is op-
tional.
It is now required quite definitely that the
filter which is normally used on a camera
should be in place during calibration. A state-
ment of radial distortion along each diagonal
is now required as well as the average of these.
A completely new section, Part 5, on devia-
tion of filters and camera port glasses has been
added. This, I am afraid, is largely a Canadian
development, but we have found tests of this
sort very useful and I think it may be helpful if
other people avoid the mistakes we made when
starting such tests,
Those are all of the important revisions. The
reporters know that there was a vote taken on
whether we should include the word ‘principal’
in the principal point of autocollimation and
principal point of symmetry. The vote was
slightly in favour of retaining the word ‘prin-
cipal', and the present revision is written that
way.
In conclusion, I would like formally to re-
commend that the draft revision dated 26th
August, 1960, be adopted by Commission I and
submitted, I believe, in the form of a resolution
at the Plenary Session of the Congress.
Mr J. A. EDEN: I have two short comments
on two brief papers, the first on calibration cer-
tificates. I contend that statements of distortion
and principal distance which are all one normal-
ly finds on a calibration certificate are not fun-
damental measures at all, and are obtained only
after various processes of distribution, interpo-
lation and waiting. These tend to hide the ac-
curacy of the measurements from which they
are derived, and I put in a plea that the cali-
bration certificate should list the basic meas-
urements of x and 8, or whatever they may be,
the angles of change of the lens and the meas-
urements on the plate to which they correspond.
There are, of course, no objections to the certi-
ficate also listing various distortions and prin-
cipal distance if the photogrammetrist is pre-
pared to pay the calibrating authority for this
work that I think he should be able to perform
himself. But it is only by knowing the actual
measures of x and 0 and the location of the
points that were measured that the photogram-
metrist can form an opinion of the adequacy
of the calibration measure and how far he can
apply these to the work he has in mind without
unacceptable interpolations.
The other comment I have on my second
paper is that I have treated on the subject of
air films. I feel that with air films there is a
tendency for people to be very conservative and
to stick to the brands and types that they are
used to without inquiring whether or not there is
available a more suitable brand or type for their
purposes. This, I think, is rather to be deplored.
It is not, for example, the case with cameras.
Here people go for the best cameras; in conse-
quence of this a great many cameras in use
today tend to come from one or two firms. The
other firms are thus continually striving to im-
prove the quality of their cameras; this is a very
healthy state of affairs of particular benefit to
the photogrammetrist. To obtain the same state
of affairs with films we need to be able to assess
the comparative performance of all the different
manufacturers’ products and brands, of which
there are legion, and so compare the perfor-
mances in speed, contrast, resolution, keeping
qualities, consistency of patches, anti-static pro-
perties, maintenance stability etcetera. I would
like to suggest that the Congress should recom-
mend the need for some international test on
how to do this work and how to publish its re-
sults for the benefit of all the users of the films
