American Cinematographer - September 1983 - 91

focusing without having to resort to

it is important to note that the smaller
image format size of %" cameras rela¬

things about the
ways in which video lenses focus

bright enough to be viewed optically,
simply projects a special pattern
brightly enough to register on the

tive to 35mm film

tube; and it does so in such

the

which reflect the fact that a video

that the

macro

a

lens.

There

are

it

other

is

traditionally operated by
one person. For example the amount
of rotation required of the focusing
ring on a video lens is likely to be sub¬
stantially less than that on a film lens.
If the camera operator is focusing as
he frames, he is likely to prefer a
focusing ring which moves from one
extreme to the other as quickly as
possible. If on the other hand an as¬
sistant is following focus during a
camera

a

way

image has an extremely lim¬
depth of focus. The pattern is

ited

such that a waveform monitor can be
used to check the

adjustment of back

focus.
Video lenses may also differ from
film lenses in the way in which they
achieve a change in focus. There are

shot with the aid of marks on the set

essentially two ways in which a lens
can be focused. The simplest method
is by moving the whole lens away
from the focal plane, generally re¬
ferred to as rear element focusing. A

and

second

markings on the lens, he will pre¬
focusing ring which moves fur¬
ther for a given adjustment in the
focus. This is one of the things that
was apparently not fully appreciated
in the initial design of some of the
fer

a

method

is front element

focusing which achieves the same net
effect without having to move rear el¬
ements at all. The front elements can
move

in different ways

depending on
design of the lens. For
fairly common in video

the mechanical

lenses for the EC-35 in which the

instance it is

focusing ring rotates through only 90°
or 100° as compared to 270° or 300°
on a good film lens.
Since a video lens is generally de¬
signed with the assumption that it will

lenses to have the front element

be focused with the aid of the view¬

finder, and since the focusing ring ro¬
tates

through a much shorter

distance; the calibration of the focus¬

ing ring will probably not be as accu¬
rate as the calibration of a film lens.

The proper focusing
also complicated by

of a video lens is
the fact that the
pickup tubes onto which the lens
must focus the image are glass tubes
mounted in rubber in such a way that
they can be replaced. The stability of
this surface cannot compare with the
stability of the aperture and pressure
plate in a film camera, and the adjust¬
ment of back focus must be a routine

part of setting up a video camera.
Some video lenses have

a

back focus

adjustment built into them to facilitate
this process. It should be remem¬
bered that it is not possible to mea¬
sure flange depth with a video camera
in the same way it can be done with a
film

camera

because of the presence

of the

prism. As a result back focus is
generally adjusted by moving the
position of each pickup tube.
There
use

are

two collimators made for

with lenses

on a

video

camera.

tate as the lens is focused. This can

conceivably create problems with
changes in astigmatism and it can
also complicate the use of certain fil¬
ters if they are mounted directly on
the front of the lens. Rotating a polar¬
izing filter or a star filter will alter the
effect that it creates.
There is

a

variation

on

front ele¬

ment

focusing which involves moving
a second group of elements in the
lens rather than the front group. This

is, of

course,

another

to increase the accuracy

focus calibrations

on

rea¬

of the

the lens-es¬

pecially if focus is set according to
depth of field charts rather than by
eye with the viewfinder.
Mechanical Construction
Another way in which video lenses
may differ from film lenses is in the
mechanical construction of the focus¬

ing and
a zoom

zoom mechanisms. Flow well
mechanism is designed and

the materials used in it

can

affect how

well the lens holds focus

throughout a
impossible to design a lens
to maximize performance for every
focal length in the zoom range, so
there are a series of compromises in
the optical design of any zoom lens. If
zoom.

It is

the

zoom mechanism is constructed
of aluminum rather than brass and
or

too loose, the optical design com¬
promises are likely to be compounded
by mechanical problems, especially
after the lens has seen a good bit of
use. Compromises in the mechanical

construction of video lenses

turing and marketing video lenses.
Some video lenses

every bit as

lens to be sealed

againt dust or mois¬

market
level of

the lens to another and the lens

can

completely sealed against con¬

Depth of Field
of the

suggestion of Mathias. Instead of
projecting an image which must be

than

Although depth of field is a function

image format and a specified
acceptable circle of confusion rather
a

characteristic of the lens itself,

are

carefully

ture. If the front group is moving rela¬
tive to the rearmost group, the interior

increasing in volume and
must take in air. If a second group of
elements is moving back and forth it is
simply displacing air from one part of

are a re¬

flection of the economics of manufac¬

lenses, but there

of the lens is

if the tolerances in

are

to achieve focus is that it enables the

device developed by
Panavision for the Panacam at the

AMERICAN CINEMATOGRAPHER, SEPTEMBER, 1983

manner

the construction of the mechanism

tamination from the air outside it.

new

son

stainless steel

things. First of all it can
minimize the change in magnification
which occurs during a focus change.
One of the complaints that film cam¬
eraman often have when shooting
video is that the lens gives them a
noticeable zooming effect during a
focus pull which they are not ac¬
customed to seeing with film cam¬
eras. The second advantage of
moving an internal group of elements

onto the tube which can be

a

this

achieve two

be

other is

that

is done in film lenses in order to

One

by Mitchell projects an image
inspected
via a viewing system in order to check
focus as the tube is adjusted. The

ro¬

cameras means

cinematographer working in video
will get a greater depth of field for a
given angle of view and f-stop.
Switching from 35mm to 2/3" video is
roughly comparable to switching to
16mm in terms of the increased depth
of field. In many instances this can be
an advantage, but in others where a
cinematographer wants to limit depth
of field for stylistic reasons adapting
to 2/3" video may be a bit awkward.
Attempting to control depth of field in

are

constructed

as

good film

others where the

simply cannot support that
craftsmanship.

Auto-Iris
Another

by-product of the fact that
operator traditionally
has not had an assistant to pull focus,
is the tendency of video lenses to in¬
clude a motorized exposure control
system. In most video productions
until recently the iris has generally
been controlled remotely by the video
a

video

camera

engineer or automatically by the
strength of the signal generated by
the pickup tubes. Since a video signal
amounts to the most sophisticated
89
American Cinematographer - September 1983

Table of Contents for the Digital Edition of American Cinematographer - September 1983
