sundance 2006 - Zoael

Lenses for Digital Cinematography
Two Approaches
BY LARRY THORPE AND GORDON TUBBS
DIGITAL HIGH DEFINITION VIDEO
cameras that capture 24 discrete
motion-images per second
(24p) are increasingly being used to
make theatrical features by traditional
film-using moviemakers as well as digital
cineastes from the TV/pro video
world. Both types of digital filmmaker
are taking advantage of the workflow
benefits and relative affordability of
HD 24p to produce everything from
major Hollywood releases to indie pictures.
Both types of filmmaker are also
gaining a better appreciation for one
another’s production methods.
Film-users have come to appreciate
the longer load times, lower media
costs, and workflow improvements that
HD 24p brings from its video-camcorder
origins. Moviemakers migrating from
the TV/video world, meanwhile, have
access to the creative options afforded
by new prime and zoom lenses designed
for digital cinematography. Like their
film-using colleagues, these moviemakers
know that lenses not only create the
optical image for presentation to the
camera’s digital sensor (or film shutter),
but are also a powerful means of manipulating
that image to enhance and convey
the art of storytelling.
TWO WORLDS OF LENSES
Lenses for digital cinematography
today include cine primes and zooms
designed for traditional film-style
shooting as well as a broad range of
portable HD lenses made for electronic
field production (EFP) and electronic
newsgathering (ENG) in the
TV/video world. Which style of HD lens
to use for 24p moviemaking usually
depends on what an individual filmmaker
is comfortable with, or what
their background (film or TV/video)
happens to be.
In the world of film—whether it be
Super16mm, 35mm, 65mm, or other
formats—directors of photography,
camera operators, assistant camera
operators, and grips all have pre-determined
expectations as to how their
equipment is supposed to operate.
These expectations are based on a
long process of experience and lenstechnology
refinements that began
more than a century ago. Professional
film crews expect certain features to
be present in the equipment they use.
TV and video professionals also have
expectations as to how their equipment
will operate. Portable TV cameras were
first developed for ENG during the
1970s. Lenses that could zoom proved
their value for news early on. EFP applications
soon followed. Thirty years later,
when companies such as Sony and
Panasonic began introducing portable
HD cameras that can be switched to
address both the 60i ENG/EFP needs of
the TV/video world as well as the 24p
requirements of digital cinematography,
the need emerged for lenses that could
address both styles of production.
Canon’s Broadcast &
Communications division, one of the
world’s leading makers of high-performance
lenses, had a long history in making
standard definition (SD) portable
video lenses, as well as 16mm and
Super16mm film lenses for Arriflex and
Aaton cameras. The transition to
designing and manufacturing HD lenses
for both digital cinematography (“cine”)
and HD ENG/EFP was a natural next
step. Canon’s first cine lenses, known as
the High Definition-Electronic
Cinematography (HD-EC) line, were
introduced in 2000, and included an
18:1 zoom lens and a wide-angle zoom
lens, which had a 9x zoom ratio. Those
lenses have since been replaced; today
Canon’s HD-EC line today includes a
series of six Cine Prime lenses, three
Cine Zoom lenses, and a revolutionary
Anamorphic Converter that is available
to be used with all B4-format lenses.
Canon has also, meanwhile, developed
a full line of versatile and innovative
portable HD lenses for the ENG and
EFP needs of the TV/video world. These
lenses include extreme wide-angle models,
lenses with built-in Image
Stabilization, and “eDrive” lenses with
programmable features (such as zoom
speeds and focus settings) that camera
operators can automate for exact, digital
servo-controlled repeatability.
INITIAL DIFFERENCES
The design parameters from an
optical standpoint are basically the
same between Canon’s HD-EC line of
cine lenses and its line of portable HD
ENG/EFP lenses, with some subtle differences
as will be outlined later.
There are, however, definite mechanical
and electronic differences between
HD-EC lenses (primes and zooms) and
portable HD lenses (all of which are
zooms). These differences are a reflection
of the long and differing traditions
in the way these lenses are used.
A first major difference between
these lens categories is evidenced in
the fact that portable video lenses
include an integral primary servo-drive
unit. This unit contains an internal
zoom motor, iris motor, and—in some
cases—a focus motor. The unit, which
also serves as a camera grip that
includes control buttons, the zoom’s
rocker-switch, and—in some models
an LED screen to set eDrive features—is
designed to comfortably
accommodate the contours of the
user’s hand. This servo-drive unit/grip
88
assembly provides a means of both
holding the camera (which is balanced
on the operator’s shoulder) and controlling
it physically and electronically.
The drive units built onto the side
of portable HD (and SD) video lenses
do not exist on cine lenses. Other than
the highly readable and detailed markings
on the lens barrel, and the gear
teeth that circle the outside of that
barrel, the cine lens is bare. This is
because traditional film-style shooting
uses third-party accessories for focus
motors, mechanical focus drives, zoom
motors, and iris motors.
Incidentally, the pitch of the gears
used for cine-zoom, focus, and iris is
quite a bit larger than what is used for
portable HD lenses. Canon uses the
international standard usually referred
to as the “Arri gear pitch,” which
allows for all standard film accessories,
including zoom motors, focus motors,
and mechanical attachments to be
married to the cine (HD-EC) lens just
as they would on a cine lens designed
for a film camera.
A second major difference between
cine and portable HD lenses is the rotation
angle of the focus barrel. Today all
portable lenses produced by Canon for
video applications use Internal Focusing
(which is also true of Canon’s HD-ECstyle
cine lenses). The amount of this
rotation is based on the fact that
portable camera users need to be able to
move from infinity to close-focusing
without taking their hand off the barrel.
This limits the rotation angle of the
focus barrel to about 100 degrees.
Cine-style shooting is, however, very
different from ENG/EFP-style shooting.
In cine-style shooting it’s typically not
necessary to move the focus barrel from
one side to the other without taking
your hand off the barrel. But the focussetting
numbers and calibration marks
engraved on the barrels of cine-style
lenses must be far more precise, readable,
and abundant. Consider: With
portable video lenses, focus is achieved
by looking through the viewfinder. This
is not true in cine-style shooting, where
focus is typically accomplished by precisely
measuring the distance from the
focal plane of the camera to the subject
and then rotating the lens barrel to the
proper focus mark. Although markings
on a video lens may take you from 10 to
50 feet in one small movement of the
barrel, this would never be accurate
enough in cine-style shooting. Cinestyle
lenses have to be marked in many
more segments, which requires a complete
mechanical re-design of the focus
system from what’s found in portable
HD (and SD) video lenses. The amount
of focus rotation is increased on the
cine-style zoom-lens barrel from video’s
100 degrees to 270 degrees of rotation
(280 degrees in cine primes). This
allows for a greater number of precise
focus marks to be engraved. These are
large, luminous markings that are very
precise and easy to read.
A third major difference between
these lens categories is that portable
video lenses are marked for focus from
the front vertex (the front element of the
lens). In other words, if the camera operator
were to measure the distance from
the camera to the person or object being
photographed, that measurement would
begin from the glass face of the lens.
Focus markings on a cine lens barrel,
on the other hand, do not indicate the
distance from the front lens element to
the subject. Instead they refer to the
distance from the film plane (indicated
by a small circle intersected by a vertical
line engraved on the camera body) to
the person or object being photographed.
Even though digital cinematography
cameras do not use film—
and therefore have no film plane—this
style of focus measurement is still used.
The difference is that instead of a film
plane, the intersected circle represents
the position of the plain of the image
sensor (a CCD or a CMOS chip).
MANAGING LIGHT IN VIDEO AND CINE
LENSES
Both video and cine lenses have a
built-in diaphragm that controls the
amount of light they transmit. This variable
aperture alters the diameter of the
bundle of light rays passing through the
lens, allowing fine control over the
brightness of the image being formed at
the lens output port. Aperture Ratio
relates to image brightness and is the
ratio of the effective aperture (D) and
the focal length (F) of the lens. The
brightness of the output object image of
a lens is proportional to the square of
the aperture ratio.
THE VIDEO WORLD—GEOMETRIC
APERTURE
In the traditional video world, for
purposes of calibration, the steps of
aperture control are termed f-
Numbers—and the nature of this control
is known as a Geometric Aperture
system. The f-number expresses the
optical speed (the receptivity to light)
of the lens on the assumption that
100% of the incident white light is
transmitted through the lens. This is
impossible in real-world lens design,
and thus the f-number is not an
absolute measurement of the lens’ optical
sensitivity. Given that the spectral
transmittance of lenses made by different
manufacturers invariably will not be
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