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FIM 1801 - FUNDAMENTALS OF CINEMATOGRAPHY
Exposure
EXPOSURE
At its most basic, the job of the cinematographer is simply to put an image on film in a
process we call "getting an exposure." Any cinematographer worthy of the title is certainly
trying to do a lot more than just that, getting an exposure and, more precisely, getting a good
exposure, is the essential bottom line of everything we do. What constitutes a good exposure?
If you ask Kodak or a lab they'll probably tell you something like a film is properly exposed when
light strikes the emulsion in sufficient quantities to trigger a chemical change in the silver-halide
crystals.
Well, that's the scientific answer. In practice of course, if every part of a frame were
"perfectly exposed" you'd have nothing except a gray frame. In any frame of film there will be
areas which are exposed correctly along with areas either under exposed or over exposed.
Portions of a frame overexpose to appear lighter while others underexpose and appear darker.
The combination of overexposed and underexposed areas, forming the light and dark portions
of a frame, gives an image its shape, form, color and texture.
The only practical definition, from a working cinematographer's point of view, is that a
piece of film is properly exposed when the image looks the way you want it to look. Sloppy as
that may sound it's the only definition that allows for the creative abuses film is routinely
subjected to in the course of getting a so called "proper" exposure. Obtaining that exposure is
a process of regulating the quantity of light which reaches the film. In exposing a shot you must
alternately switch your thinking, from considering specific areas within a frame as being either
over, under, or perfectly exposed, to thinking of the entire frame as one single exposed object.
Over Exposed
Under Exposed
Properly Exposed
EXPOSURE TIME or SHUTTER SPEED
Motion pictures create the illusion of motion by rapidly flashing a series of still images.
Each frame of film is flashed at least twice on a movie screen to reduce perception of flicker.
The length of time that each frame of film is exposed to light is called the exposure time. Some
cinematographers refer to this as shutter speed.
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FIM 1801 - FUNDAMENTALS OF CINEMATOGRAPHY
Exposure – Page 2
At sync speed, film is traveling through the camera at the rate of 24 frames per second.
This means that each frame is being exposed for 1/24th of a second. For one half the time, the
film is being moved in and out of position in the gate and for one half the time it is being
exposed to light. One half of 1/24th of a second is equal to 1/48th of a second. For
convenience this is often rounded to the nearest tenth making it 1/50th of a second. This is the
amount of time that each frame of film is being exposed to the light and is referred to as
exposure time or shutter speed. Therefore, at sync speed, the standard exposure time (shutter
speed) for all motion picture photography is 1/50th of a second.
STANDARD EXPOSURE TIME at 24 fps is 1/50th of a second
Camera Speed
If we were to run the film through the camera faster than 24 f.p.s., as is commonly done
to achieve a slow motion effect, than the film would spend less time in the gate and our
exposure time would be reduced proportionally. Doubling the camera speed, from 24 f.p.s. to
48 f.p.s., would allow the film only half as much time in the gate and our exposure time would
be reduced in half, from 1/50th of a second to 1/100th. In this way camera speed directly
effects exposure time.
Shutter Angle
The SHUTTER is the part of a camera or projector which acts like a "light valve," either
blocking light or letting it through. In most movie cameras shutters are circular disks, partially
solid, partially open, which rotate in a position between the back of the lens and the film.
Movie camera shutters spin in sync with the film being moved in and out of the gate so they
block light while the film is moving and let it through while the film is at rest.
The portion of a shutter which is open and allows light to pass through is measured in
degrees to give us our shutter angle. Most movie cameras have a shutter that's half solid, half
open. Measuring the open portion of the circle we get a shutter angle of 180 degrees (180°).
Many cameras have fixed-shutters on which the open area can not be changed, but most
professional, 35mm movie cameras have variable-shutters which allow you to adjust the
shutter angle. Changing the shutter angle will effect how long the film is exposed to light. If
the shutter angle is reduced in half, from 180° to 90°, then the film would be exposed to light
for only half of the time it's in the gate. Like camera speed, shutter angle directly effects
exposure time.
The two factors which directly effect exposure time in cinematography are the CAMERA SPEED
and SHUTTER ANGLE.
To get your EXPOSURE TIME - Know your CAMERA SPEED & SHUTTER ANGLE.
exposure time
shutter angle
360 x frames per second
=
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FIM 1801 - FUNDAMENTALS OF CINEMATOGRAPHY
Exposure – Page 3
The following illustration of the shutter is from Film Production Technique, Creating the
Accomplished Image, Third Edition by Bruce Mamer (Wadsworth/Thomson Learning, 2003)
EXPOSURE INDEX, ASA or ISO
All motion picture film is sensitive to light in varying intensities. The measurement of
how sensitive a particular film stock is to light is called its Exposure Index (EI) number. It may
also be referred to as ISO. The larger the EI/ISO number, the more sensitive the film is to light,
and the less light it needs for a proper exposure. The smaller the EI number, the less sensitive
the film is to light and the more light it needs for a proper exposure. Similar to the series of fstop numbers, there is a series of EI numbers used to rate the film's light sensitivity.
The standard series of EI/ISO numbers are:
. . 12, 16, 20, 25, 32, 40, 50, 64, 80, 100, 125, 160, 200, 250, 320, 400, 500, 650, 800, 1000 . .
By looking closely at this list of numbers you will see that the values double every three
numbers. This is because each EI value is equal to 1/3 of a stop. This means that every three
numbers equals a change of one full f-stop. A change of one full f-stop either doubles the
amount of light or cuts it in half. So, by doubling or dividing in half the EI number, it is the same
as doubling or dividing in half the amount of light. As an example, the same amount of light
that gives you an exposure of f/4 at EI 200 will require an f/5.6 at EI 400 or an f/2.8 at EI 100.
The film’s manufacturer based on extensive testing of the film determines the Exposure
Index (EI). This number is what the manufacturer feels will give the "best" exposure of the film.
Each film can label will show the recommended EI rating for the film stock, for both Daylight
and Tungsten Light. The ultimate decision on what EI to rate the film is up to the Director of
Photography, and is usually based on his experience in using the particular film stock. Other
terms that are used to refer to the Exposure Index (EI) are ASA, ISO or DIN.
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Exposure – Page 4
KEY EXPOSURE TERMS FROM VOICE & VISION
Elements of Exposure
Light Meter
Incident Light
Reflected Light
Incident Light Meter
Reflected Light Meter
Shutter Speed
Taking a Reading
Photosphere
Spot Meter
Middle Gray
Through The Lens Meters (TTL)
Auto Iris (Auto Exposure)
Manual Exposure Control
Zebra Stripes (Zebras)
Gain
Electronic Shutter
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