Different DPs have different methods
of calculating exposure and communicating with the gaffer and assistant cameraperson
who usually sets the stop on the lens. Here are some of the systems. In the
motion picture business most lenses are calibrated in "T" stops which are calibrations
for the actual amount of light a lens transmits.
Some DP's use a BASIC STOP which
is related to ASA and scene brightness or an incident light reading. A SHOOTING
STOP takes into consideration an increase or decrease in exposure dictated by
"effects" filtration, shutter angle, and camera speed.
The Basic Stop is determined by the
film's rated speed, possible modified development (pushing), "flashing" and/or
a color correction filter if used, such as an 85. The DP may also choose to
under or over expose the negative. The assistant does not make these decisions.
Any filters, ND's, shutter angle changes or speed changes are taken into account
by the assistant, who makes the exposure calculations. (Hopefully the DP also
passed through the assistant stage and also knows how to make these calculations.)
Calculating the Shooting Stop from
the Basic Stop is sometimes the assistant's job. There are meters that make
all the calculations, but I think that is dangerous. I found that if both the
assistant AND I made the calculations, on paper if necessary, we rarely made
Once all the corrections are understood,
a meter, calculator or charts can help, but roughly checking the plusses and
minuses of each exposure factor can prevent mistakes.
Example: Basic stop is f 16, at 96
FPS we loose 2 stops to f 8.0, for a strobe effect and sharper image the shutter
is closed to 22 degrees, (3 stops less) gives f 2.8. We have an f 4.0 zoom so
either faster film is required or a forced development to raise the ASA by one
stop to f 4.0.
Filtration, in almost all cases,
reduces exposure. Fog filters on high key scenes and "flashing" can increase
Shutter angle: a 180 degree shutter
gives a 1/48 shutter speed, usually called a 1/50. Many Panavision cameras have
a 210 degree shutter that allows 1/5 stop more light.
To calculate shutter speed: a 180
degrees shutter means the film is exposed 1/2 of the time (the film is moved
to the next frame during the other 180 degrees.) At 24 frames the film receives
a 1/2 of 24 = 1/48 second of exposure.
Camera Speed changes (FPS) can translate
into + or - stops.
24 FPS = "0" correction. 12 FPS adds
one stop exposure (Stop down lens = go to next higher F number.) 48 FPS reduces
exposure one stop. Open the lens one stop, one less F number.
If the DP uses a graduated neutral
density filter to darken a sky or part of a scene, this is not usually entered
into the exposure calculation.
INCIDENT LIGHT READINGS
For many years in Hollywood, each
film had a foot candle rating for a certain F Stop. The Spectra Incident Light
Meter was calibrated in foot candles and F stops. For F STOP READINGS, a slide
for each different ASA adjusted the INCIDENT light hitting the photo cell so
the needle read directly in F stops. This of course assumed the scene was of
average brightness and lit by the same light the reading was taken in.
For FOOT CANDLE READINGS, the DP
reads the key light, and incident reading which is his BASIC exposure. All other
readings were related as ratios to the key light. The system worked well and
still does especially for KEY, FILL and KICKER lighting of average interior
sets. Outside most scenes are average brightness, in the same light and can
be exposed only considering incident light from the sun, sky and reflections
from objects. Problems arise when the scene is a source of light, such as a
sunset where only a reflected light reading will work.
With the advent of spot meters, designed
for still use some DPs consider both incident and reflected light.
Some DPs use the Zone System. (See
Some DPs using the Minolta meters
read stops in numbers followed by tenths. For example 8 "and" 5 tenths is half
way between 8 and 11. They always say "and" between the stop and fraction to
avoid problems with "two eight" being 2.8 and not 2 and 8 tenths, which is almost
Other DP's round out the difference
between stops in 1/3's, which accurate enough. A DP also needs control of other
variables such as actual film ASA, film development, and light meter calibration
to be this accurate.
Some DP's rely on the Spectra IVa.
With a flip of some switches, it will measure stops, footcandles, or lux. It
indicates shutter speeds as FPS or fractions of a second. Best of all, all the
information is displayed on a backlit LCD screen at one time. This can be a
time saver, but I suggest that both the DP and the assistant understand how
to make the calculations by hand. If all meters fail and it's sunny, the "Sunny
16" rule could be a basis for keeping shooting.
Knowing something about all these
systems will prepare the assistant for whatever system a DP uses.
It is very important to establish
what a DP means when they say "give me a little under 8". He could mean F 7
if he meant a lower F number which is more exposure OR... he could mean F 9
(a smaller F number) which is less exposure.
There are also corrections necessary
for dark objects such as a black cat in a coal bin. If an incident reading is
taken with no correction there will be little detail. Most DPs know to give
a dark scene more exposure with an incident reading. For a reflected reading
he wants a black cat and not a gray one which the reflected reading would give
and would under expose.
For a white cat in the snow it is
the opposite. For an incident reading less exposure is needed to retain some
detail in the scene. For a reflected reading more exposure is needed so the
snow and cat records white and not neutral gray.
Thanks to Jon Fauer and Bill Bennett
for contributions to this piece. Check out Jon's books and tapes on Arriflex
cameras which include crucial information on assisting and shooting at www.fauer.com.
© Copyright 1999-2004 Ron Dexter.
All Rights Reserved