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DISCUSSION ON 
but the actual cables of a normal telephone or 
power transmission line, one of 5500 volts. 
QuEsTION: What you mean is not a 33,000 
volt overhead transmission line but — as we 
would say, a house service line. 
Mr OLSON: Yes, sir. 
QUESTION: In England this would be de- 
scribed as high power. 
Mr OrsoN: That is correct. It is a wire. I 
can definitely say by taking my pencil; probably 
none in the back of the room can see this small 
section at the end of my pencil; I would guess it 
is in the neighbourhood of 1/8th of an inch 
diameter. We resolved a wire that size. 
CHAIRMAN: I think Mr Olson means to say 
that the photo interpreter's appetite for defini- 
tion is insatiable. Mr Jackson would like to add 
something. 
Dr JACKSON: The reason that someone 
detects a wire as Mr Olson discussed can be 
identified on a photograph, if he will permit me 
to differ with him, is not because the photograph 
was able to resolve it but because of the very 
characteristic I spoke of, the light-spreading 
capability — I shouldn’t say capability because 
it has a deterrent effect — the light-spreading 
effect of the line and the emulsion. The light 
reflected from the wire or from railroad tracks 
or things of that kind has a specular aspect so 
that the contrast of that object is much higher 
than it would be if it were just being illuminated 
in a non-specular fashion. That is one aspect 
that makes it possible to see such a thing; an- 
other one is the continuous nature of the thing 
that is being observed. If you were to look at a 
little piece of such a wire or railroad track you 
might not see it because of the statistical char- 
acter of the emulsion in destroying the effect of 
the image, but if the end you are looking for is 
continuity, continuity sufficient so that you can 
fill in the gaps, then it becomes a thing that can 
be seen. 
I would like to say that I have seen a pho- 
tograph taken from an altitude of 150 miles over 
the Mexico/United States border where you 
can see a fence which is on the border that you 
could not possible resolve in a photograph. 
CHAIRMAN: Relating to the subject of Mr 
Jackson's paper but not exactly covered by it is 
another factor which is of some importance. The 
PRESENTED PAPERS 123 
problem has arisen in addition to the problem 
of measuring quality from the point of view of 
the lens manufacturer and film manufacturer. 
How does the photo interpreter express the 
quality of photography. He is — as you noted 
in this discussion — he is particularly interested 
in the information that it conveys and I would 
like to throw out for comment by Mr Bousky 
and some of the rest of you a suggestion which 
was recently put forward by a couple of people 
in the States that a possibility might be to assign 
a simple number to the photograph, this number 
being the number of times that a photographic 
image from the negative should be magnified 
without beginning to lose information. We are 
all familiar with this in a small scale pho- 
tograph. You look at it with your eye and you 
can see so much. You magnify it twice and you 
can see a little more and so on. This would ob- 
viously have to be carried out on the original 
material and the first generation material and 
the effect was that if you get to certain point of 
magnification, 10 times perhaps, or 15 times, 
with a better photograph, or 20 times with a still 
clearer photograph, you come to the point 
where you can no longer see any more informa- 
tion and from then on you can see grain or 
degradation. I would like to know therefore 
whether you think this is a faulty approach to 
the subject. 
COMMENT: Well, this is, I think, a good 
coarse approach to the problem. 
CHAIRMAN: I am glad of that word. 
COMMENT: From the standpoint of trying 
to pin things down specifically, the magnification 
alone is inadequate because the human behav- 
iour tends to outline a certain amount of field 
when one looks at an object, and to just magnify 
a portion and throw away a lot of — I must be a 
bit more specific — if you magnify and main- 
tain the same angular field, you are throwing 
away a considerable portion of the photographic 
field which in fact reduces one’s ability to recog- 
nize detail, so that the two have to be considered 
together. About all I can say is that this magni- 
fication number concept is a good coarse ap- 
proach to the problem but would be inadequate 
for a specific definition. 
Other CoMMENT: I feel that it is even coarser 
than indicated, that you restrict yourself only 
to this angular field. This would vary according 
to the nature of the object you are looking at. 
Most probably the interpretation of some types