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Exposure Meters
By Jim Branch & Nasir J. Zaidi, ASC Member (Associate)
The usual final adjustment of a motion-picture camera for exposure control
is made with the iris diaphragm in the camera lens. While this is a very
simple adjustment, a great deal depends upon its accuracy. Much thought has
gone into the objectives to be attained by the adjustment of the diaphragm,
and the means to obtain a correct adjustment.
It is recognized that a prime object of exposure control in motion-picture
photography is to obtain consistent and uniform images of the principal
subjects. It is very important to obtain flesh tones which will be
consistent from one scene to the next. It is undesirable to have flesh tones
which will be light in one scene, dark in the next without reason, and
again light in the next scene. Correct exposure control will provide
negatives which are consistent from scene to scene and can be printed on a
very narrow range of printer lights.
Modern exposure control is based on the use of a good light meter. The light
meter measures the effective intensity of the light, taking into account
the sensitivity of the film in the camera and the exposure time. The
exposure time is a result of the frames-per-second rate at which the camera
operates, and the angle of the shutter opening. Professional
cinematographers usually think in terms of 24 frames per second and a
175-degree shutter, which give a basic exposure time of 1/50 second. The
light meter combines all of the foregoing factors to give an answer in terms
of the appropriate camera lens stop.
Never in the history of motion picture photography has the need for accurate
exposure control been as great. With the increasing use of digital video
postproduction to manipulate original motion picture photography, the
cinematographer more than ever needs to monitor how the exposures generated
on the original negative as densities will reproduce as digital electronic
signals. Maintaining control of creative photographic visions can be greatly
enhanced with the aid of a highly accurate exposure meter. The meter should
be able to predict how original scene luminance will ultimately reproduce as
print densities empowers the cinematographer to confidently explore maximum
creative possibilities.
Light meters are of two types. Some measure the incident light which
illuminates the subject. Others measure the light which is reflected from
the scene. The results obtained from the two different types may be quite
different. It is important therefore to understand the differences between
the two types.
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Incident Light Meters
These meters are normally used at the location of the photographic subject.
They measure the light which is effective in illuminating the subject. They
give an answer in terms of f-stop or T-stop for the camera lens. The camera
lens diaphragm opening is then set to match the effective intensity of the
prevailing illumination.
When the film is exposed, the various reflectances presented by the subject
will then each fall into a given place in the film acceptance range. For
example, a face tone of 30% reflectance will fall into the 30% reflectance
position in the film acceptance range. This method thus provides
consistently uniform face tones from scene to scene.
The incident light meter accomplishes its purpose by doing two things. It
measures the incident light intensity at the location of the photographic
subject. It also takes into account the conditions of illumination geometry;
that is, whether the subject has front key light, side key light, or a back
key light. The meter combines these factors and gives an answer in terms of
the correct setting for the camera lens diaphragm.
There are several makes of incident light meters which use a
three-dimensional light collector. The hemispherical light collector allows
these meters to perform automatically the dual function described above.
These incident light meters are normally used at the position of the
principal subject, with the hemisphere pointed at the camera lens. The
hemisphere then acts as the miniature face of the subject. All illumination
which will be effective on the subject, including key light, fill light,
line light, hair light, eye lights, etc., will be received, evaluated and
integrated by the meter. The meter will then indicate directly the correct
f-stop or T-stop for the camera lens. Incident light meters are
particularly useful because they may be used on a scene before the principal
subject appears. They may also be carried through a scene, with the
hemisphere always pointed at the camera lens, to detect uneven
illumination, and particularly hot spots, into which the subject may move
during the action. This allows the scene illumination to be suitably
balanced before the principal subject is at hand.
In the case of outdoor photography, it is not always necessary to take the
meter to the location of the principal subject. Under such conditions the
illumination is usually uniform over considerable areas. If the illumination
is the same at subject location and at camera location the meter may be used
at camera location. Care should be exercised to point the meter in the
proper direction, as though it were at the subject location.
Exposure meters, in general, are either analog (with a needle) or digital.
The introduction of the analog incident meter with the 3-D light-collecting
hemisphere revolutionized the method of determining proper exposure for the
cinematographer.
Today, a number of companies throughout the world manufacture exposure
meters employing the basic incident type principles in their design, but all
due credit should be given for the invention to Don Norwood, ASC, who
patented it, and Karl Freund, ASC, who was instrumental in the development
and design of the original “Classic” Spectra light meter. Most incident
meters are provided with suitable adapters so that they may be converted for
use as a reflected light meter if the occasion should so indicate. The
reflected light adapter can be used in a situation where the cinematographer
encounters difficulty in putting the meter into a position to read either
the illumination directly on the subject, or illumination similar to that on
the subject. Such a situation, for example, might be encountered when taking
a picture out of the window of an airliner in flight. The reflected light
attachment can also be used in other situations to evaluate the relative
brightness of a background.
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Special Effects
When a special effect is desired, the cinematographer may use the incident
light meter to first determine normal exposure for the subject. Then he may
then deliberately modify that value, up or down, to achieve the desired
effect. This can be done with considerable confidence because the incident
light meter will give a firm foundation upon which to base the desired
modification.
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Specific Situations
There are some situations, occasionally encountered in outdoor photography,
which require special attention.
1. Unusually light or dark backgrounds are cause for consideration. When a
scene includes an unusually light background, the cinematographer may wish
to first use the meter as an incident light meter to determine the basic
exposure for the principal subject in the foreground. Then he can convert
the meter to a reflected light meter in order to measure the brightness of
the unusual background. The second reading is then used to modify somewhat
the basic incident light reading. The same procedure could be followed in
the case of an unusually dark background.
2. Outdoor scenes that include a subject in the foreground as well as
distant objects, such as mountains, in the background, usually also include
considerable aerial haze, which may be invisible or only partly visible to
the eye, but strongly visible to the camera. A frequent photographic result
is a recording of the aerial haze overlaid on the scene background. This
would give the appearance of an overexposed background. It is recommended
that in such a situation a haze-cutting filter be used to improve the
background. In addition, use the procedure previously described for the
case of an unusual lighting background.
3. Scenes consisting of a mixture of sunshine and shade areas, with the
principal subject in a shade area, can be handled by: (a) using the meter in
the sunshine area, or (b) opening up the lens by 1/2 to 2/3 f-stop from the
meter indication.
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Reflected Light Meters
Reflected light meters can be classified into two groups, according to
function. The meters in each group may give exposure readings which are
substantially different from those given by the meters in either of the
other two groups. This is due to differences in basic principle of
operation.
Group 1. These are the meters which are designed to measure the
average brightness of an entire scene. Such meters are usually used at
camera location and pointed at the scene. For a discriminating observer,
this method appears to give acceptable results only in the case of a very
limited category of scenes, those which have front-lighting and a foreground
subject of medium tone as well as a background of medium tone. In other
types of scenes, which include side-lighting or backlighting, or very bright
or dark backgrounds, or large areas of sky, the exposure results are
questionable. This is because the meter, when used by this method, is
affected not only by the unit brightness of each portion of the scene, but
also by the relative area of each. Thus a large area of sky would influence
the meter to dictate a small lens aperture which might result in an
underexposure of the face of the principal subject in the foreground. Any
backlight may strike directly into the meter cell and cause an unduly high
reading on the meter. This also would result in underexposure of the
foreground subject. Large bright backgrounds tend to cause meter readings
which result in underexposure of foreground subjects.
Large dark backgrounds tend to cause meter readings which result in
overexposure of the foreground subject. If this method is used it should be
considered only as a very rough guide, subject to considerable modification
according to the experience of the cameraman.
It is interesting to note that this method is the one generally used in the
built-in automatic exposure control systems of amateur motion-picture and
still picture cameras. It has been noted by many that the photographic
results do not meet the high standards of professional cinematography.
Group 2: These are the spot meters. A spot meter may be used at
camera location and aimed at a selected spot in the scene. The effectiveness
of the meter is heavily dependent on the operator's judgment in the
selection of the spot. The selected spot must be precisely representative of
the particular combination of elements, which compose the scene. In the use
of such a meter the operator must be particularly careful when confronted
with a scene that presents strong contrasts between the selected spot and
the scene background. An example of such a situation would be a case where a
person in the foreground is in front of a very light background, such as sky
or white buildings, etc. In such a case the operator should modify the spot
reading provided by the meter according to his own estimate of the
situation. When the use of a reflected light meter is required, the results
of determining the exposure can be greatly improved by using a "Kodak
Neutral Test Card."
This card is a piece of sturdy 8"X 10 "cardboard that is neutral gray on one
side and white on the other. The gray side reflects 18% of the light falling
on it, and the white side reflects approximately 90%. Also, the gray side
has a protective lacquer overcoat that reduces specular reflectance and
resists damage due to fading, fingerprints, soil, etc. To a light meter, an
average scene is one in which the tones when averaged form a tone brightness
that is equivalent to middle gray -- a tone that reflects 18% of the light
illuminating it (the same tone and refiectance of the gray card). When a
scene is not average the gray card as a reference helps you make the proper
exposure judgments. A Kodak Gray Card is manufactured under close tolerances
to provide a neutral gray-side reflectance of 18% ( +/- 1% ) and white-side
reflectance of approximately 90%.
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Testing
Small errors may exist in meters, lens calibrations, emulsion speeds and
development. These small errors will frequently cancel out without undue
harm to the final picture. It is when these errors add up in the same
direction that their cumulative effect is serious. It is wise, therefore, to
test equipment, film and meters under simulated production conditions so
that errors may be detected and corrected before production begins. It is
always a good idea to "tune up to the variables."
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Exposure Meters- Incident Type
The Specifications of the exposure meters listed below are the meters
designed exclusively for the cinematographers and therefore have included
functions such as f/stop, footcandles/lux which are necessary for Cine
photography.
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Spectra Professional IV-A
Type: Spectra Professional IV-A Advanced Digital Exposure Meter (ADEMTM) for
measuring Incident and Reflected light. Reflected light acceptance angle is
42 degrees-square.
Technology: For highest accuracy and reliability it utilizes Advanced front
end op amp, Custom Sealed Hybrid
electronic circuitry, Multiple Range Linear Circuit (MRLC) controlled by
Advanced microcomputer and custom liquid crystal display with Backlit
Electroluminescent lamp.
Light Sensor: Silicon Photovaltaic cell , Computer selected glass filters
tailored to spectral response of the film.
Swivel head assembly turns 270 degrees.
Measuring Capability: Direct readout of photographic exposures. Measures
contrast ratios and average in f-stop and illuminance modes. Measures
Illuminance level in footcandles (fc)and lux (lx).
Measuring Range: One million to one (20f/stop) direct reading Multiple Range
Linear Circuit. Digital f-stop: f.35 to f128 in 1/10-stop increments. Analog
f-stop: f0.7 to f45 in 1/3-stop increments. Photographic
Illuminance: Footcandles 0.1 <![endif]>to 70,000 and lux 1 to 100,000.
Contrast Ratios: Calculates ratio of key plus fill to fill light alone from
1:1 to 0.2 <![endif]>999:1. f/stop- difference: Plus or Minus 12 f/stop.
Average: Average of two readings and or continuous averaging 0.3 <![endif]>in
f-stop and footcandle mode.
Display Range: ISO: 3 to 8000 in 1/3-stop increments. FRAMES PER SECOND: 2
to 360 (for motion picture cameras). Exposure time: 1/8000sec to 30 minutes.
Resolution: Digital 0.1-stop. Analog: 0.2-stop
Accuracy: Digital 0.05 f-stop. Calibration is traceable to National
Institute Of Standard Technology (NIST),
Washington, DC.
Additional Functions: Memory Recall switch is to recall stored reading in
memory (m1), current reading in memory (m2) and /or the Ratio, Average and
f/stop difference of the two readings.
Instant Recall Of All Measurement And Calculations: By pressing the recall (RCL)
switch, the first reading
(M1), the second stored/memorized reading (M2, contrast ratio or brightness
(f/stop) difference, average footcandles or average f/stop, can be recalled.
Immediate Update Of All Exposure Calculations: If the film speed (ISO),
frames per second (FPS) or exposure time settings are changed, the f/ stop
are immediately recalculated and displayed in normal, f/stop difference, or
average modes.
Memory Erasure: You can erase the stored/memorized readings in memory (M1)
and memory (M2) by pressing the store (STR) and recall (RCL) switch
simultaneously.
Camera Shutter Angle: Complete list of shutter angles and equivalent
compensation table conveniently provided in the instructional material.
Bright Green Display: Electroluminescent lamp for viewing displayed
measurement at very low light level.
Power Consumption: Operating 5 mA/reading. Data Retention 5uA.
Power Source: One 6 volt battery no's: A544, or PX28L, or PX28.
Estimated Battery Life: Approximately one year with normal use.
Dimensions: 5-1/2 X 2-1/2 X 2. Weight: Approximately 6 ounces
Accessories Supplied: Spectra meters are supplied with Photosphere (for
incident light), Photodisc (for key or
directional light measurement), Leather carrying case, Neckstrap, Battery,
instructional manual and warranty card.
Warranty: Two year limited warranty. Specifications subject to change
without notice.
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Spectra Professional IV
Type: Hand-held exposure meter for measuring incident and reflected light.
Light Sensor: Silicon Photovoltaic cell, computer selected glass filters
tailored to spectral response of the film. Swivel head 270 degrees.
Measuring Capability: Direct readout of photographic exposures. Also
measures illuminance level in footcandles and Lux.
Measuring Range: One million to one (20 f-stops) direct-reading
multiple-range linear circuit controlled by
microcomputer.
Display Range: ISO film speed: 3 to 8000 in 1/3 stop increments.
Camera speed: 2 to 360 frames per second. Resolution: Digital: 0.1 f-stop.
Analog: 0.2 f-stops.
Accuracy: Digital: 0.05 f-stop.
Additional Functions: Memory store and recall.
Lamp: Electroluminescent lamp for backlit liquid crystal display.
Power Consumption: Operating (reading) 5mA. Data retention 5uA.
Power Source: One 6-volt battery. (A544, PX28L or PX28).
Estimated Battery Life: Approximately 1 year with normal use.
Dimensions: 51/2"X 21/2"X 2". Weight: Approximately 6 ounces.
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Reflected Light Meters (Spotmeters)
The Specification of the Reflected Light Meters listed below are of the One
Degree Spot measuring type.
Pentax One degree spot meter
Measuring System: Spot measuring of reflected light.
Exposure read-out: LED digital display of EV Numbers with two dots (each of
which equals +1/3 EV).
Photosensitive Cell: Silicon Photo Diode.
Measuring Angle: 1° (small circle in the viewfinder)
Measuring Range: (ASA/ISO 100) EV 1 ~ 20 (EV 20 displayed as “O”)
Scales: ASA/ISO 6 ~ 6400; Aperture f/1 ~128; Shutter speed, 1/4000 sec. ~1
sec. ~ 4 min.; Cine Index (24
frames/s); IRE,1 ~ 10.
Measuring Distance: From about 1.5m to infinity, fixed focal length
(focusing down to about 1m possible screwing the eyepiece out as far as it
will go).
Viewfinder: Pentaprism type with unreversed, upright image. Viewfinder
coverage similar in shape to that of a
35mm SLR, and a diagonal field of view of 26°, horizontal field of 17°, and
vertical field of 14°, viewfinder magnification of 1.15x, adjustable
eyepiece of –2 to +1 diopter.
Power Source: One 6V silver oxide battery or equivalent alkaline battery.
Battery Check: failure of LED digital display to illuminate indicates
battery needs replacing.
Others: Equipped with Tripod Socket and 40.5mm filter thread.
Dimensions & Weight: 45.5mm (W) x 144mm (H) x 83mm (D). 250 g (without
battery).
Accessories: Case, wrist strap battery and lens Cap.
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Minolta Spotmeter F
Type: Spot-reading reflex-viewing exposure meter for ambient and flash
light.
Measuring method: Reflected light by silicon photocell detector masked for
1° angle of acceptance.
Optical system: Through-the-lens reflex type utilizing semi-silvered mirror
and pentaprism: Focus fixed for readings 1.3m to infinity, with optional
close-up lens 0.6 to 1.4m.
Field of view: 12x17 with 1° area marked by circle in finder.
Magnification: 1.4 x
Measuring range (at ISO 100): Ambient: EV 1.0 to 22.5
Flash: f/ 2 to 90+0.9stop
Display range: ISO 12 to 6400 in 1/3-stop increments
Shutter speed (ambient): 30 min to 1/8000 sec. In 1-stop increments (cine:
1/50 sec.)
F number: f/0.7 to 90+ 0.9 stop in 1/10-stop increments
EV numbers: -4.3 +28.5 in 1/10-stop increments
Brightness difference: -9.9 to +9.9 stops in 1/10-stop increments
f-number, EV number, and brightness difference shown in both external and
finder
displays. Analog scale (f-number): 1.4 to 45 in ½-stop increments.
Other indication/displays:
Analog and digital display readouts change automatically to reflect
ISO/time-input
Changes; “S”, “A” or “H” on internal display indicates exposure is
calculated for shadows, average (midtones), or highlight areas respectively
“star” mark appears when reading on digital display is found for taking
brightness- difference measurements; flash mark appears when using flesh
mode.
Brightness difference indication:
When “star” mark appears in external/ finder digital displays, difference in
brightness between calculated
measurement
and subsequent readings is shown in 1/10-stop increments; calculated reading
displayed again when measuring button is released.
Other functions: Memory: 2 channels, both indicated by pointers on analog
display: digital recall possible;
measuring clevel adjustment screw: tripod socket.
Repeatability: 0.1 EV
Power source: One AA battery
Operation Temperature Range: -10 to 50° C (14 to 122° F)
Storage Temperature Range: -20 to55°C (-4 to 131°F)
Dimensions (W x H x D) : 48x150x89mm (1-7/8 x 5-7/8 x 3-9/16in.)
Weight: 240g. (8.5 oz.) without battery
Motion Picture Screen Brightness/ Luminance spotmeters are included in
this section simply for the benefit of the cinematographers who are
interested in the quality of the projected print.
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Spectra Cinespot One Degree Spot Meter
Type: Through-the-lens viewing spot-reading automatic/manual luminance
meter.
Light Sensor: Silicon Photovoltaic cell with 1° angle of acceptance.
Viewing Optics: 1.6X magnification, erect system with focusing eyepiece.
Measuring Capability: Direct readout of luminance level in foot lamberts or
candelas.
Measuring Range: Low Range 0-30 FL (or 0-100 cd/ m2) readings legible down
to 0.5FL. High Range 0-300 FL (or 0-1,000 cd/m2), upper limit may be
increased by use of accessory 10X or 100X attenuators.
Spectral Response: Within +/- 4% (by area) of CIE Photopic Luminosity
Function.
Accuracy: +/- 1% of full scale or +/- 5% of reading (whichever is greater).
Error Due To Chopped Light: +/- 0.5 % at 24 cycles/ second.
Power Source: One 6-volt battery. (Eveready 544 or equivalent).
Estimated Battery Life: Approximately 1 year with normal use.
Dimensions: 5" X 2"X 6.4" Weight: 15 ounces.
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Minolta Luminance One degree spotmeter
Type: Reflex-viewing spot-reading automatic/manual luminance meter.
Light Sensor: Silicon Photovoltaic cell with 1° (1/3° in model nt-1/3°) of
acceptance.
Viewing System: Focusing through-the-lens reflex type. Objective lens 85mm
f/2.8. Angle of view: Circular 9° with central 1° (1/3° in model nt-1/3°)
marked circle. Magnification: 2.96X focused at infinity.
Measuring Capability: Direct readout of luminance level in footlamberts or
candelas.
Measuring Range:
Model ft-1°: 0.01 to 99900 ft-L (0.01 step)
Model nt-1°: 0.1 to 99900 cd/m2 (0.1 step)
Model nt-1/3°: 1.0 to 99900 cd/m2 (0.1 step)
Display Range: Red (+) LED's at the right of the number display indicates
10X and 100X the display reading.
Accuracy: Within +/- 4% of C.I.E. standard +/- 1 digit in last display
position.
Screen-flicker accuracy: Within 1% of average luminance with projection
cycle of more than 72 Hz and duty of 7% (projector at 24 fps).
Analog Output: Output voltage: 1V over full scale. Output impedance: 10
kilo-ohms.
Power Consumption: 6 mA in analog mode. Meter can monitor changes in
luminance for a period up to 40 hours.
Power Source: One 9-volt battery (Eveready 216 or equivalent).
Estimated Battery Life: Approximately 1 year with normal use.
Dimensions: 2 7/8" X 6 3/8" X 4 11/16"
Weight: 18 1/8 ounces, without battery.
Much of the material in this section of the manual is basic, but reference
should be made to Don Norwood, ASC, Karl Freund, ASC and Eastman Kodak
Company for the gray card information.
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