Boxoffice-August.16.1965

J
MAXIMUM SCREEN
ILLUMINATION
Continued from page 28
a few in the 60- to 70-foot size. The trend
has continued on in recent years, and we
now have screens of 65. 70. 75. and over
100 feet in size. In addition, 70mm projection
and Cinemascope projection have
made possible satisfactory projection with
high powered arc lamps and greatly improved
projection lenses. Manufactmers
of projection lamps and projection lenses
have certainly met the challenge for more
screen illumination for greater screen area
in drive-ins and de luxe indoor theatres.
And there has been a continued improvement
in lenses for increasing light and
better picture definition, even with shortfocal
length lenses.
For better projection, it is of the utmost
importance that the picture projected have
good overall focus. Better definition in recent
years has been obtained from more
highly developed optical glass in lenses
ani from added refinements in lens manufacture.
Now modern makes of projection
lenses have sharp enough definition, flatness
of field, and freedom from color
fringes.
The short focal length lens is a boon to
the architect in the modern theatre concept.
There are some problems, however,
in its use in some cases. It should be understood
that a shorter focal length lens
witliin a given series has a smaller aperture.
Entrant and exit angles of these
lenses are wider than tho.se of longer equivalent
focus.
Curved surfaces are more critical, more
steeply curved and with the entire assembly
closer to the film. Being closer to the film,
the slight changes of the focus knob have
a greater percentage effect relative to the
film. This greater effect is responsible for
the more critical focus. Another factor is the
lens speed, which, in a properly designed
lens, will admit light at a wider angle.
This wider angle adds to the problems of
achieving sharp focus, of course. Primarily,
the problem stems from the short focal
relationship of film and lens, not to the
design or speed of the lens. To prove this,
experience has witnessed the exchange of
a normal lens of a specified speed with that
of a faster lens, but with a built-in magnilier
iKoUmorgen built-in Magna-com magnifier),
using a longer focal length backup
lens. When fui'ther from the film, the
entire result is less critical, and depth of
focus is restored.
DEPTH OF FOCUS FACTORS
Note: Theoretically depth of focus is
related primarily to speed, but in practice
it is also related to closeness to film, due
to film buckling in gate. The greater the
distance from the film (lens), the more
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Stable the picture quality, edge to edt
say optical e.xperts.
Use of short focal lenses is necessafor
35mm widescreen projection in new
theatres: short focal lenses are now ava
able in l^ii-inch focal length and will d
liver a picture that will have satisfacto
focus sharpness. Again, one should ke
in mind, when short focal lenses are oi
ployed, they are critical to focus but l
projectionist can, by being on the j(
maintain, in most situations good pi
ture definition with high quality leii-si
GOOD OVERALL FOCUS
|
Using short focal lenses necessitati
having a mechanism equipped with
I
heavy lens mount, so that the lens wi
be held rigid in order to maintain a go|
overall focus. Many poor definition pro]
lems have been traced to poorly construe)
ed lens mounts and the lens barrel ni
held rigid. The lens holder should be maso
that it will envelop almost all of ti
lens barrel and have a strong clampii
aiTangement for holding the lens tlgi
and free of vibration. Most modern pr
jectors have a one-piece clamp for belt
optical alignment. The accui'acy of alig:
ment, both in lens clamp and one-pie,
lens barrel, is essential to good pictu,
definition and easier focusing. Mode
projector mechanisms are equipped wi,
vei-y fine focusing mechanism, so that o
can secure a sharp focused image.
Early pictures were shown in total dar
ness. and many screens had a large blai
area paitited flat black around them. Tl:
produced a brilliant picture, due to tl
large black border and small pictui'e are
Today, however, the auditorium has sul
dued lighting and widescreen projectio
This makes it necessary to use ( 1 ) mo
powerful projection lamps, i2) lenses th
help increase the light output and i'
high gain screen surfaces, but still w
duce pictures without glare that can I
viewed comfortably by the patron. The!
has also been some improvement in shu
ter design and higher speed Intermittei
movements — producing increased seres
illumination. In addition, rapid strid^
have been made by carbon manufacture
in making carbons that also produce mo
and better light.
MATCH REFLECTOR, LENS SPEED
We do not want to overlook that fa
that the limit has about been reached
the trend toward "faster" projection lens
and the fact that faster lenses are mo:
critical to focus in many situations. In
provement has been made in the quali'
and speed of reflectors in arc lamps. Tl
speed of reflector and lenses should 1
"matched" as closely as possible for sati:
factoi-y results. The speed of reflectors
generally printed on the reflector for re..
erence.
,
When pmxhasing new lenses, it is a