Cinematography-Theory-And-Practice

Figure 16.1. (previous page) A complexsetup for a timeslicing shot.(Photo courtesy of PAWS, Inc.)FLICKERAs discussed in the chapter on Lighting Sources, there are two basickinds of light sources. One is a filament (usually tungsten) that isheated by electrical current until it glows and emits light. Theother type is a discharge source. These include fluorescents, HMIs,Xenons, mercury vapor, sodium vapor, and others. In all of these,an arc is established between a cathode and an anode. This arc thenexcites gases or a plasma cloud, inducing them to glow. All dischargesources run on alternating current.Any arc-based bulb powered by alternating current has an outputthat rises and falls as the waveform varies. Alternating current risesand falls as it heats a tungsten filament as well, but the filament stayshot enough that the light it emits does not vary a great deal. Thereis some loss of output, but it is minimal, usually only about 10 to15%, not enough to affect exposure. With discharge sources, thelight output does rise and fall significantly throughout the AC cycle.Although rarely perceptible to the eye, flicker appears on film as anuneven variation in exposure. This effect is a result of variations inexposure from frame to frame as a result of a mismatch in the outputwave form of the light and the framing rate of the camera. Flickercan be bad enough to completely ruin the footage. The output ofan AC power source is a sine wave. When the current flow is at themaximum (top of the wave) or minimum (bottom of the wave) theoutput of the light will be maximum: the light doesn't care whetherthe flow is positive or negative.When the sine wave crosses the axis, the current flow drops to zero.When that happens, the bulb produces less output. Since the lightis “on” for both the positive and negative side of the sine wave, itreaches its maximum at twice the rate of the AC: 120 cycles persecond for 60-hertz current and 100 cycles per second for 50-hertzcurrent. For an HMI with a magnetic ballast (a copper coil woundaround a core), the output at the crossover point may be as low as17% of total output.With film there is another complication: the shutter is opening andclosing at a rate that may be different than the rate at which the lightoutput is varying. When the relationship of the shutter and the lightoutput varies in relation to each other, each film frame is exposedto different amounts of the cycle. The result is exposure that variesenough to be noticeable.There are two possibilities: the frame rate of the camera can beunsteady, or the frequency of the electrical supply can fluctuate orthe frame rate of the shutter creates a mismatch in the synchronizationof the shutter and the light output. The first two are obvious:if either the shutter rate or the light output are random, it is clearthat there will be different amounts of exposure for each frame. Thethird is a bit more complex. Only certain combinations of shutterspeed and power supply frequency can be considered acceptably safe.Deviations from these combinations always risk noticeable flicker.Four conditions are essential to prevent HMI or fluorescent flicker:The first two conditions are satisfied with either crystal controls onthe generator and camera or by running one or both of them fromthe local AC mains, which are usually very reliable in frequency.cinematography308