American Cinematographer - August 2019 - 14

A Professional's Notes for
Amateurs, Part VII
"Discoveries of della Porta and
Others in Primitive Photographic
Research Related"
by Joseph A. Dubray, ASC
AC May '27

Joseph A. Dubray, ASC served for a number of
years as AC's technical editor.

The camera has not been a stable
article, and the cinematographer, in the
course of events, has found it necessary
to discard the old outfit for one of
advanced design, the old outfit being a
total depreciation.
And any depreciation at all on the
cinematographer's camera outfit is likely
to be out of proportion to their salary, as
is evident when it is known that it is reliably estimated that upwards of
$300,000 are represented in camera
equipment owned by cinematographers.
As compared with directors and
principal members of the cast, cinematographers' salary is indeed decidedly out of proportion, if they are
expected to invest thousands of dollars
as well as contribute unlimited mental
and physical effort. There is a question
as to whether the cinematographer has
done the wisest thing, after all, in
supplying that which the producer
should legitimately furnish.
The cinematographer, working on
freelance pictures, is placed at a great
disadvantage, since income is spasmodic
and of uncertain quantity. The equipment, however, must be up-to-date or
at least meet the requirements of the
fastidious producer.
August 2019

A lens can be defined as "any
transparent medium bounded by regularly curved refracting surfaces (one of
the surfaces may be a plane)." The
curvatures of the refracting surfaces are
calculated so as to force all of the incident rays to meet at a certain point,
which may be behind the lens (convergent lenses) or in front of it, on the imaginary prolongation of the refracted rays
(divergent lenses).
It is evident that any transparent
and isotropic substance may be used for
the making of lenses, and that each
surface of a lens may be shaped as a
portion of a curved solid, of well defined
size and of regular geometrical form.
Lenses are named according to
their shape, so if their surfaces are
portions of spheres, they will be called
"spherical," the same appellation applying when one of their surfaces is plane.
If the surfaces are portions of
cylinders, the lens is called cylindrical.
If one of the surfaces is a portion
of a sphere and the other portion of a
cylinder, the lens will be called "spherocylindrical" and so on. Lenses may be
parabolical, toroidal, etc., according to
the shape of their bounding surfaces.
In photography, only spherical
lenses made of glass are used.*
The spherical form has been
chosen because of the relative easiness
in which such shapes may be obtained
with the extreme exactness that is
required, and glass is the material used
because of its transparency, its resistance to extraneous influences and its
chemical stability.
Generally speaking, glass is a
compound of silica with metallic oxides
and two main divisions are derived from
such combination.
Crown Glass is a compound of
silica and lead, with soda or potash, or
both.
ASC 100th Anniversary

During the latest years, new
elements have been incorporated in
glass in order to change its molecular
composition and adapt it to the exact
requirements of modern optical instruments.

[*2019 Ed. Note: In the decades
since this was originally written, photographic and cinematographic lenses
have incorporated non-spherical lenses
- namely aspheres - as well as lenses
made of materials other than glass, such
as plastic and fluorite.]
A Professional's Notes for
Amateurs, Part IX
"Convergent and Divergent
Lenses; Points and Planes; and
Other Data Considered"
by Joseph A. Dubray, ASC
AC July '27

It is a well known fact that if a
convergent lens is placed facing the sun,
a certain position will be found at which
a small and extremely brilliant spot will
be seen at a certain distance from the
lens. Furthermore, if the hand is placed
at this distance from the lens so that the
brilliant spot rests upon it, an impression
of heat is immediately felt, which
becomes rapidly unbearable for its
intensity. If the spot is made to fall upon
some quite inflammable material, such
as paper, dry straw, cotton, etc., a relatively short exposure to its heat is sufficient to ignite the material.
The point of extreme maximum
heat is called the "focus" of the lens.
Focus is from the Latin word focus, which
means "fire."
Let us consider a bi-convex lens as
in Fig. 20 [see page 16].
The rays emanating from the sun
and incident to the first surface of the
lens may be considered as being parallel
to the axis of the lens, due to their
extreme distance from it.
Following the laws of refraction,
these rays will be refracted at their
entrance into the lens, and refracted
again at their emergence from it.
In other words, all the rays paral-

American Cinematographer - August 2019

Table of Contents for the Digital Edition of American Cinematographer - August 2019