34 THE PHOTOGRAPHIC IMAGE, DISCUSSION
Mr Welander’s experiments on image imotion.
I have had a quick look at this paper, which
was published in the Congress, and obviously
the logic of the situation is that we should ap-
ply the frequence response methods right
through the photographic system, including all
the elements, the atmosphere and image motion,
For my part, it is much too early to comment
usefully on that.
I would like to ask Mr Welander whether
there are any up to-date measurements off axis
on photographic lenses in Sweden?
Mr E. WELANDER: The measurement method
is published by Professor Ingelstam in Sweden
and we have only measured on the axis.
Mr G. C. Bnock: This seems to me a weak-
ness in our approach at the moment, inasmuch
as most people are more interested in the off
axis performance than the on axis, which after
all is where the lens is at its best. We are getting
to the point where the on axis performance can
almost be predicted from the aperture. There-
fore, we cannot really claim we are doing much
with frequency response measurements until
the off axis measurements are a matter of course.
I hope this situation will change in the next year.
As far as we are concerned I know it will, but
it is slow progress.
Mr Carman raised the question of test ob-
jects in resolution measurements. This is cer-
tainly a very difficult subject. I feel we have been
very illogical over the years; the test object which
we have used in this country has been a his-
torical accident. There was no very good reason
at the time this was introduced, it seemed a tole-
rably good thing to do but the reasons were not
very strong. I think one could make out cases
for other approaches, the long lines on the one
hand as in the more recent national bureau of
standards test chart have many attractions just
because they are long lines, equally the Cana-
dian annulus for the very opposite reason — that
it is about as far from the line as you can get —
is equally attractive, depending on what you
want to find out.
Personally, I have gone through a phase
where I was very strongly attracted to the long
lines from the point of view of doing micro-
densitometry on them, but anyone who has had
experience of this kind of approach will share
my own doubt as to how we interpret micro-
densitometry on long lines. The question of
grain averaging seems an extremely complex
one and only when you do it do you realise how
very complicated is the act of vision, of looking
at the test object and saying whether it is re-
solved or not. In some mysterious way the eye
and brain seem able to know that the grain is
there and to integrate it out, whereas the objec-
tive microdensitometer approach records all the
grain; if you make the lines so long that the
grain is integrated out one is not quite sure what
one is doing. It is very different from looking
at a piece of detail in an image. Therefore, I
feel that we are still all novices in this field.
My own preference, of course, would be to
standardise on low contrast test objects. At least
let us reduce the number of variables we have
in our standard. However, Mr. Cruset has in-
dicated that he would not like this for several
reasons: one is the point I mentioned earlier on
that some detail is of very high contrast. He
mentioned particularly North Africa in his own
experience. Another reason is that there are a
lot of high contrast tests in existence. It is not
much trouble to them in addition to low con-
trast, and then you have two points on a curve
in any case, which is probably better than one.
In that connection I cannot help referring
though to Mr Carman’s work on the contrast
of ground detail as distinct from details seen
from the air, which, speaking from memory, is
less than 0.2 contrast on the average. This again
seems to me to be a very strong argument for
always keeping to a low contrast test object, but
these are personal things we will have to argue
out in a small committee.
I am not quite sure that I agree with Dr
Washer’s comments that all is well. I think this
is still a very difficult situation and we must
be alive to all the possibilities continually. At
least with these discussions we are approaching
somewhat nearer to a fuller understanding of
the requirements of testing photogrammetric
cameras, and that is the main thing.
[ see it is time for me to stop.
The discussion was continued in the next
meeting.
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