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Obviously, this reasoning is not in itself sufficiently
developed to be entirely satisfactory, but it serves
to .orrfir* the results of care{ully controlled experiments.
Mn. Bar-r,: Are there any further questions ?
Mn. Gmrrrtr: Mr. Rayton spoke, I believe,
of there having been no radical improvements in
lenses for the last twenty years. I was under the
impression there had been developments of great
importance in optical glass that would improve
lenses.
Mn. RavroN: I know of no fundamentallv
new optical glass that has been produced in that
length of time.
il4n. Gnrrurrr: Do you think that there will be
any production of higher grade lenses?
Mn. RavroN: I believe it is true that the 1906
catalogue of the Schott Glass Works contained
glasses that are not found in their t92B catalogue'
They have since eliminated some glasses that were
found to be unstable or otherwise unsatisfactory, so
that the ones they manufacture today are all reliable.
I do not believe there is any fundamentally
important glass available today that was not available
twenty years ago.
Mn. Gnrnrrmr: Not knowing anything about
the studios, I feel at liberty to ask questions. With
reference to the depth of focusing: I have heard it
said that a certain amount of hardness of {ocus in
the fore ground and a soft back ground was desirable
as u *.utt. of attracting the attention of the
spectator to the important actor' or the props' in
the picture, and leave the rest in a hazy back
ground, so that you do not see the rest o{ the setonly
the important people.
Ii{n. RavroN: With Mr. Griffith's permission,
I refuse to be drawn into a technical discussion on
that point, and will leave it to the cinematographers
to discuss.
Mn. Duenav: The character of the set in question
would govern the amount of depth of {ocus
that was required for the best efiect.
Mn. Barl: There is one question I would like
to ask Mr. Rayton with reference to the desirable
amount of difiusion. What is the effect when you
are using a diffusion disc, or is the difiusion disc
independent of the stock or the lens ?
Mn. RavroN: The efiect of the diffusion disc
will not be independent o{ the aperture' The correction
o{ a lens for spherical aberration is never
perfect. In a high speed lens the departure from perfection
may be considerable (illustrates on blackboard).
According to the experience of the designer
and the efiect he may be aiming to produce,
he decides just how he will dispose of the residual
uncorrectible remnants of spherical aberration. It
often happens, in high speed lenses, that closing the
diaphragm appears to spoil the definition of the
image on the ground glass and that re-focusing is
necessary to obtain best results. with this and other
effects in mind, it is really not easy to decide just
what is the best disposition to make of the spherical
zones in a fast lens. This subject has received considerable
attention in optical literature. Results of
this theoretical reasoriing are not always confirmed
by experiment.
[At this point a test film made at Metro-
Goldwyn-Mayer Studio, showing results of tests
conducted to discover the focus shifts if any due
to change from Arc to Mazda illumination was run.]
Mn. RavroN: I wish to say in the original projection
viewed at the Metro-Goldwyn-Mayer Studio
the efiect was very clear and satisfactory. I
personally appreciate the efforts of the gentlemen
who have put this picture on the screen under discouraging
difficulties. It serves the purpose of showing
how the test was conducted, even if the quality
of the projection is not so good as we would like.
The distance from the object in best focus is about
13 feet. The nearest object was about 8 feet, and
the most remote about 35 {eet from the lens.
Mn. Dunnav:
'Were they focused under White
Arcs ?
Ms.. Ravrox: We focused the lenses with
Mazda illumination in each case.
Mn. Gnrrrrrrr: I want to ask an unrelated
question regarding the care of closed lenses, particularly
the polishing operation in order to clean
up the surface of a lens. What is the proper technique,
Mr. Rayton ?
Mn. RavroN: A soft, well-washed linen cloth
with a little rouge and water constitutes satisfactory
equipment for.cleaning a lens if the rouge can
be depended upon to be free from grit and is not
allowed to become hard and dry. IJnless the best
of optical rouge is obtainable and used intelligently,
it is better to omit it. Pure grain alcohol (not denatured,
for sometimes this will stain a lens surface,)
is excellent, but care must be taken not to
let the alcohol come in contact with the lacquer on
the lens mount. Blow as much of the dust ofi the
surfaces as possible before touching them. IJse as
little pressure as will suffice to clean the surfaces.
For loose lcnses like condensers, etc., pure soap and
water is efiective, rinsing thoroughly in running
water and drying with a soft, clean linen cloth.
Avoid chamois and silk.
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