Download Report - Academy of Motion Picture Arts and Sciences

distance from the screen in the line of projection is
of influence.
One more subject remains {or consideration
which, I understand, has caused some uneasiness in
cinematographic circles. There seems to be some
belief and some fear that with Mazda lighting and
Panchromatic film the lenses now in use cannot be
made to give satisfactory results, because, it is said,
the best photographic focus does not coincide with
the best visual focus. We shall first inquire into
this on theoretical grounds and then report on such
few controlled experiments as have been made to
subject this matter to test.
In No. 31 of the Transaction o{ Society o{ Motion
Picture Engineers, L. A. Jones published curves
setting forth, graphically, data on the average radiation
{rom Incandescent Tungsten lamps of wattages
such as are used in studio work, the transmission
characteristics of a representative photographic objective,
and the sensitivity curves of Orthochromatic
and Panchromatic emulsions. The product for any
given wave length leads to a quantity which Mr.
Jones has called photicity.
L
b
d
9
wavrlEncri
Figures 7 and 2 show the results of multiplying
together these three curves for the two kinds of
fift. They show the relative response of the emulsions
to the various wave lengths emanating from
Incandescent Tungsten after allowing for the filtering
effect of one of the standard photographic lenses.
The two curves are not comparable, quantitatively,
since some of the data on which they are based are
not expressed in the same units in both cases. They
serve, however, to show that both emulsions respond
to all wave lengths without either of them falling
to an insignificant value until a wave-length is
reached corresponding to the orange region o{ the
spectrum. In the blue, activity begins at wavelength
330. Below this point, no matter how much
radiation there might be within reason its effect
would be insignificant because of the absorption of
the lens. The principal difference in the curves is,
of course, the extension of sensitivity into the red
displayed by the Panchromatic film.
Any failure of visr,ral and photographic foci must
depend upon the combination of values shown by the
(mL,
E
E
l
WAve€ncrh
FIGURE
curves of Figures 1, and 2 and upon the condition
of correction of chromatic aberration of the lens in
question. Classical practice in correcting a lens {or
chromatic aberration requires that the best focus
Ior a wave length at about 4340 in the blue-violet
region be united with the best focus for a wave
length o{ about 5890 in the yellow, and that the
focal lengths or magni{ying powers be practically
equal for those two colors. With this practice agreed
upon as standard there is room for some difierences
in final correction because of the fact that exact
coincidence in size and position of the violet and
yellow images is possible only for one zone of the
lens. The outer zones of fast lenses cannot be
given the same correction as the central zone and in
the exact disposition of these residual aberrations
there is some chance for differences in performance
between different makes of lenses.
Assuming, however, that the best focus for the
two colors mentioned is coincident there remains a
residual color aberration due to which the colors
lying between the two chosen colors will come to
focus somewhat nearer the lens and colors lying beyond
the chosen wave-lengths will be united in focal
points lying {arther from the lens than the normal
focal point. This secondary spectrum, as it is called,
might conceivably result in a shift of the apparently
best photographic focus if it were large enough and
if the emulsion had sufficient selective effect. Panchromatic
emulsion, however, appears to utilize the
same range of wave-lengths as the Orthochromatic
plus some greater sensitivity in the red. The actual
secondary spectrum efiect in the range added to the
efiective photographic radiation by the use of Panchromatic
film is actually very small because of the
low dispersion of glass for the long wave-lengths of
red light. It does not exceed a fourth of the region
of uncertainty as to what constitutes the best visual
focus and amounts to a very small part of the variation
of Chromatic aberration between central and
marginal zones in anF:2.5 or F:2.0 lens.
There is undoubtedly sufficient secondary spectrum
in long focus lenses of low relative aperture
to be detected photographically especially when
using filters, but with the short focus high aperture
II
(mru)
| \\ |
L'"
J