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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
