24 THE PHOTOGRAPHIC IMAGE, BROCK
Sensitive emulsion. The “Howlett philosophy” is of course to use whatever
emulsion is used in practice. In the past this has been Kodak Aero Super XX or the Ilford
equivalent, in Anglo-Saxon countries. This is the weakest point in the entire framework
of the resolution test, because it is not possible to specify a standard emulsion which will
always be available, and with improvements in technology the one-time standards will no
longer be used in actual air photography.
Even among current emulsions there appear to be measurable differences in thres-
hold characteristics between the nominally equal commercial products, and in such a
complex product as a photographie emulsion it would not be strange if one manufacturers
product varied in turbidity and graininess from time to time. Now however, we are faced
with the withdrawal of Super XX and its replacement by two quite different emulsions,
Plus X and Tri-X, one of higher and the other possibly of lower resolution. How can con-
tinuity be preserved, since it is not possible to continue the manufacture of an obsolete
emulsion for testing purposes? It would certainly be possible to select a different kind of
emulsion, e.g. Panatomie X or Ilford High Resolution, and start testing again from a
new point, but there is no guarantee that this status could continue for more than a few
years. This is a serious problem, which seems to make true standardisation impossible. It
will be highly desirable to quote, for every camera or lens test, the limiting emulsion
resolution as measured with a microscope objective capable of very high performance.
(For low contrast testing, minor differences among objectives of the highest quality
would not be significant.)
Exposure and processing should call for no special comment, but experience of these
has not been too satisfactory. It has proved very difficult to have sufficient care taken
in various laboratories to ensure density, gamma and fog being held within reasonable
limits. There is no technical problem, but precision in photographie operations seems
hard to come by.
Interpretation of results. Ideally, the images should be examined under
the magnification which gives maximum resolution, thus involving a certain amount of
trial and error at each change of size. In
practical control of production, where the
AAN TA x
z^ |. {| | | | | resolution will fall within a known size
2o | À | | (LENS N21| | NS bracket, it has been found better to specify
YS TXT TT rivera T CE] a definite magnification and even a speci-
1.6 rL— \ EMULSION V-G fic binocular magnifier and form of illumi-
(a alo GAMMA so | nation. The difficulties of agreeing on a
= | \ Ed | common standard of “resolution” are well
9 TTY known. Added to the inherent difficulty of
= 1.0 * \ fom deciding when an asymptotic curve reaches
O 0.8 bY % oe fd its axis (Fig. 16) they limit the precision
© o6 o N pb f 1 | | of resolution testing to a rather low order.
N N | | Washer’s suggestion of using long line tar-
o4 FON NU TET gets and doing microdensitometry to esta-
0.2 Belt blish a more objective relation between con-
>) | | |
9 LL} Tmo xx) trast and frequency has not, apparently,
o 20 40 60 80 100 been developed. It has great attractions, but
Frequency in lines per mm. comes very near to frequency response,
which may be the better solution in the
Fig.16. (After Washer) Variation in long run.
contrast for increasing line frequency in
the image, for type VG emulsion and 4.3.
three target contrast.
Will the resolution test survive?
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Archives 4
