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EN: Mr Brock's excellent 
tention in photogrammetry 
lications dealing with con- 
This paper gives proper 
lties of measuring contrast 
been said that this function 
ire and that is true, but this 
applying it if it helps us to 
ity. We need high precision 
|-qualified physicists to spe- 
ut those few institute which 
ipply and are already apply- 
of this new quality criterion. 
are the publications already 
ongress, ones by Mr Brock, 
ky and Dr Meier as well as 
and an impressive publica- 
>r and Mr Hempenius meas- 
rrinters at the I T C. By the 
| is that the dodging proce- 
ample of an element in the 
m which cannot be describ- 
f resolution. Only when you 
rast transfer theory does it 
evaluate the performance of 
hat is clear because dodging 
ie area of larger details not 
nimum size) detail, but just 
etween very large and very 
lications in photogrammetry 
re, but it is clear that photo- 
y just begun to take advan- 
insmission, not only for the 
ng of lenses but also as an 
or all individual steps in the 
the judgement of image 
this fact I might propose that 
institutes who are already 
aged in contrast transfer 
hotogrammetry should keep 
maybe in a small working 
order to avoid unneccessary 
avoid duplication of tedious 
| promote the exchange of 
1rement results. Finally, such 
ould come to the next Con- 
DISCUSSION ON PRESENTED PAPERS 39 
gress in 1964 with more definite proposals after 
more experience in this field has been gained. 
Mr G. C. Brock: Keeping for the moment 
to the question of contrast transfer functions, 
there is one point I would like to make straight- 
away, which is an apology to Professor Schwi- 
defsky. I said yesterday afternoon that I had 
not yet seen any off-axis measurements of 
contrast transfer functions. This was strictly 
true, but only because I had not explored fully 
the great mass of paper we were given the other 
day, and as Professor Schwidefsky pointed out, 
his paper giving just such curves was in the mass 
of paper if I had dug sufficiently deep. I would 
add that these are very interesting results and 
I think one might claim perhaps already, as a 
tribute to the value of contrast transfer meas- 
urements, how clearly they show the improve- 
ment of the new Pleogon over the old. 
I think possibly one ought not to put too 
much to the fore this business of correlating 
resolution tests with transfer function results, 
because surely the whole point of the contrast 
transfer approach is to give you more infor- 
mation, and whether or not in happens to agree 
with the resolution test is rather by the way. 
As compared with the Stockholm Congress, 
I do get over-all an impression that we have 
moved on and everybody is much more alive 
to the things that make for good image, and 
we are not by any means standing still in this 
field. 
On Dr Meier's paper, which again I have 
only been able to read very quickly in the train 
this morning, I am slightly surprised to see that 
he allows such a big factor, he will allow a 
movement of 1.5 times the distance resolved in 
the absence of movement before this has an ef- 
fect on quality. We have always worked roughly 
to the relation worked out by Selwin many years 
ago which was a factor of about 0.6, and we 
have actually taken photographs, many years 
ago, with 1.2, I think — speaking from memory 
— which showed quite an appreciable blur, but 
that is a detail we can discuss later. 
Reverting to Professor Schwidefsky’s paper 
on the effect of photographic light, this was a 
very interesting compilation of the available 
evidence. I must say I agree with everything that 
is said, in fact it is slightly embarrassing to find 
how often and how well I agree with Professor 
Schwidefsky on these matters. But I would add 
one thing which is merely an extra: one of the 
problems in this subject is, of course, to find 
what sort of function relates the solar altitude 
with the effect of photographic light. As every- 
one knows, this used to be done many, many 
years ago in terms of the illumination on a 
horizontal plane, but Jones and others in Ame- 
rica more recently showed that this was quite 
wrong for ground photography, and they pro- 
duced the so-called luminous density function 
which shows much less variation between 20 
degrees and 90 degrees in solar altitude than 
had previously been allowed. In fact, there is 
practically no varation in their function between 
those two angles. This has been carried over 
into air photography without any good reason. 
It seems to work tolerably well on some kinds 
of terrain, but a few years ago we received 
complaints that exposure tables based on this 
luminous density function were given gross over- 
exposure in desert areas. 
We made a special study of this a couple of 
years ago in which an aeroplane flew from dawn 
to sunset over an area of desert and by usual 
photogrammetric and photographic techniques 
the effective photographic light as operative for 
illuminating the desert was determined, and this 
was found to agree (within experimental error) 
with the illumination curve and not the luminous 
density curve, and our exposure tables are being 
modified accordingly. 
This is quite a complicated subject, but when 
one goes into it hardly any other conclusion 
could have been reached, for the case of a 
desert which is a uniform plane and not a series 
of planes at all angles as assumed by Jones and 
Condit. I have passed a copy of this paper to 
Professor Schwidefsky who might be interested. 
Professor K. B. Jackson: I feel a bit like a 
foreigner this afternoon, after so much talk 
about contrast transfer because I am afraid my 
approach has been the old practical form and 
I have a lot of things that bear on a lot of things 
that have been said from a slightly different 
point of view. My time is limited, as all our 
times are, and if I get through what I am sup- 
posed to in the time I am supposed to use, I 
am going to apply at the next Olympic Games 
in the 100 metres Open because I think I will 
be in good shape. 
In spite of the effort on my part to get my 
paper here in time to be distributed, it did not 
get distributed in time to reach you. Conse- 
quently I am not referring to something you 
have seen, and therefore I am very appreciative 
of the chance that Ilford gave me locally to 
make some slides and present a pictorial repre- 
sentation of my paper in a hurry, so I will gallop 
on.