sundance 2006 - Zoael

Lenses for Digital Cinematography
CONTINUED FROM PAGE 88
the same, any two such lenses having
the same f-number may actually have
different optical speeds. This needs to
be carefully accounted for in any sideby-side
tests between different lenses
(using appropriate light meters).
The true merit of the f-number calibration
is its precise control over
changes in light level. The f-number
values are expressed as a geometrical
series starting at 1 with a common
ratio of the square root of 2, as follows:
1.0, 1.4, 2.0, 2.8, 4.0, 5.6, 8.0, 16.0,
22.0, 32.0
Each doubling of the number indicates
a precise halving of the previous
amount of light exiting the lens-output
port. This form of light control has well
served the television world. All of
Canon’s lenses (both SD and HD)
intended for traditional “video” shooting
have their adjustable aperture ring
clearly marked with f-numbers.
The Maximum Relative Aperture of
a lens is a measurement of the lens
optical speed when the lens diaphragm
is wide open—and this f-number may
(and often does) fall between two of
the standard f-numbers listed above.
In such cases, the lens is marked with
a non-standard f-number, e.g., f1.9 in
the case of the Canon HJ11ex4.7B
portable high definition lens.
THE CINE WORLD—PHOTOMETRIC
APERTURE
The filmmaking world has always
been concerned about knowing the
amount of light reaching the film emulsion.
Accordingly, their lenses have traditionally
employed an alternate calibration
system known as Photometric
Aperture—which rates lens light control
in T-numbers. T-number values
take into account the actual transmittance
of the lens and can be related to
f-Number according to:
T-Number = f-Number x 10 /
Transmittance Efficiency (in percent)
Consider a “video” HD lens with a
maximum aperture of f 1.9 and a transmittance
of 84%. The equivalent Tnumber
for that lens is 1.9 x 10 / ( 84 =
19/9.16 = T2.1. Any two lenses having
the same T-number setting will have
identical optical speeds.
DPs accustomed to using film cameras
are just as desirous of knowing the
precise amount of light reaching the
image sensors of digital cameras (particularly
if they have occasion to use one)
and hence they expect that the cine
lenses for such cameras should also be
calibrated in T-numbers. Accordingly,
Canon’s family of 2/3-inch cine prime
lenses and cine zoom lenses all have
their adjustable aperture rings clearly
calibrated in T-numbers.
90
THE CINE LENS: A VARIATION ON A
THEME OF THE HD VIDEO LENS
Those long accustomed to shooting
with motion picture film cameras have
acquired personal, subjective prefer- ences on perceived picture sharpness
and color reproduction that are often at
variance with those who shoot with digital
24p cine cameras. Quantifying the
difference in technical specifications
has proven elusive—but various efforts
are underway to attempt to do so.
Canon undertook a project to begin to
address this issue. They did so on the
basis that a broad family of HD video
lenses had been firmly established to
support conventional HDTV (ENG,
EFP, studio, and field) shooting, and
that the development of this lens technology
also afforded them the opportunity
to develop a parallel family of HD
cine lenses. These HD cine lenses,
Canon determined, would perform in
ways more tailored to the imaging
expectations of the long-established
motion picture film community. It is
important to note that the basic HD
optical design of the two families of lenses
is identical, but that some additional
subtle modifications have been added to
the cine lens family.
Having heard a variety of comments
from those using the first-generation
HD cine lenses, Canon elected to consult
with several DPs to closely examine
the issue of subjective picture sharpness
Canon HD Electronic Cinematography HJ11x4.7B KLL-SC Zoom lens and
HJ11x4.7B portable HD ENG/EFP Zoom lens.Note the focus markings on the
Cine lens and the servo-drive unit on the ENG/EFP lens.
and color reproduction in the case of a
new-generation design of an HD cine
zoom lens and an accompanying family
of cine prime lenses. The collaboration
got underway using a 1080/24p HD camera
and a range of existing HD lenses to
shoot and then subjectively assess a
range of selected images and scenes.
Even with the HD camera image
enhancement set to minimum, there
emerged a general consensus that the
visual perception of sharpness on the
HD monitor was still somewhat “hard”
(too sharp)—especially on close-ups
with the lens aperture setting near to
wide-open—compared to the equivalent
reproduction from a 35mm motion
picture film print. In addition, the DPs
urged a tight match between the color
reproduction of the new family of cine
prime lenses and the cine zoom lens to
facilitate frequent and rapid lens changing
during a shoot without the need to
invoke the auto color balance system of
the HD camera.
A considerable amount of viewing—
on a large screen—followed the shooting
tests. All of the picture detail information
of 1080-line HD is approximately
100 Line-pairs per millimeter
(Lp/mm). There was general agree-