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DETERMINING THE ASA/ISO RATING OR EXPOSURE INDEX OF A VIDEO CAMERA
When you are setting up a video camera at the beginning of a shoot, it is
possible to establish an effective ISO/ASA rating or exposure index for the
camera by means of a simple procedure that takes only a few minutes. First
set up a chip chart and light it evenly just as you would for
color-balancing the camera. Then focus the camera on the chart and open the
iris until the crossover chip is a 55 units on the waveform monitor or the
peak white chip is a 100. Normally you will have already done this in order
to set levels.
Check the f-stop on the lens, and then take a reading with a Spectra
Professional IV or IV-A incident meter at the chip chart. The photosphere on
the meter should be pointed at the camera. Set the frames per second on the
meter to read 25 FPS. Set the film speed (ISO/ASA) on the meter to 100 and
take the reading, change the film speed (ISO/ASA) on the meter until the
f-stop reading on the meter coincides with the f-stop at which the lens is
set. You have determined the effective exposure index of the camera. You
need only to remember the meter setting ISO and FPS, keep that setting on
the meter and use it as you would with film.
This method of setting up the meter to coincide with the camera has the
added advantage of compensating for any light loss due to the camera optics.
It is the equivalent of establishing T-stops for the lens, since what you
are doing is matching the meter to the amount of light that is actually
reaching the pickup tubes. The f-stop on the lens then becomes an accurate
indication both of exposure and of depth of field.
The exposure index determined in this manner may not be a scientific measure
of the camera’s sensitivity, but it is a reliable, practical basis for
setting the f-stop during production. It enables you to expose every scene
so that midtones will be consistently reproduced or to adjust the exposure
so that the production of the midtones will be altered in a controllable
manner.
It may be helpful to review how using an exposure index of this sort differs
from setting exposure simply by means of the waveform monitor. A waveform
monitor is like a reflected light reading. It can tell how to expose a scene
so that as much as possible of it can be encompassed by the latitude of the
camera, but it cannot easily ensure that a given object will be reproduced
with consistency.
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Normally a waveform monitor is used by a video engineer to set an f-stop
that will place the brightest object in the scene at peak white in the
signal. If the engineer knows that the brightest object is something like a
window where detail is not important, he or she may adjust the exposure and
let the brightest area exceed the normal peak white level in order to bring
out more detail in the shadow areas; but nothing on the waveform monitor
indicates how much to adjust the exposure. Similarly, there is no guarantee
that the brightest object in a given scene should necessarily be a peak
white. Depending on the content of the scene, the brightest object may be a
skin tone or some other object that really looks better at a such lower
level.
The waveform monitor also cannot easily ensure that an object will be
reproduced consistently from one shot to the next. Consider as an example a
scene involving two different angles in a room that has dark paneling on the
walls and is filled with dark furniture. One angle includes a white marble
bust of Napoleon next to the bookcase. The other angle does not include the
bust but includes a table and part of a wall visible to the first. Suppose
that the brightest area in the second angle is a portion of the polished
tabletop that is reflecting light from an off-screen window. The same
tabletop is visible in the first angle, but is would obviously not be a
bright as the white marble bust.
If the f-stop for each angle were set so that the brightest area was at peak
white, then the paneled wall would appear much darker in the first angle
than in the second. This might be all right for each shot out of context,
but if the two are cut together in one scene the result would be distracting
and confusing. The solution is obviously to set the exposure so that the
tabletop or the wall is more or less at the same exposure level in both
angles. Accomplishing this without an incident light meter would require
analyzing the display on the waveform monitor in order to determine which
portion of the signal represented the wall or the tabletop and matching that
level for both shots. While this may be possible, it is not very practical,
especially since using an incident light meter will give identical results.
With an incident light meter it is only necessary to take a reading with the
photosphere pointed at the camera for each angle and set the f-stop
according to the meter. In many situations it may be useful to refer to the
waveform monitor after setting the f-stop. If the scene has an extended
brightness range, it may be useful to check the waveform monitor to see how
well the range of the scene is being captured. Adjusting the exposure level
may be one only of altering the tone reproduction for that scene in order to
deal with the extended brightness range. The use of gamma compression
circuits on the camera (if it is so equipped) may be another.
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