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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