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Flash Duration Explained
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PostPosted: Mon Mar 12, 2007 2:29 pm 
Flash Duration Explained

Many studio flash users are confused by the implications of flash duration with regard to stopping action and by the effect of different power levels on flash duration.

Power Level Vs. Flash Duration:
The light output of a flashtube is not a sharp on/off function. The light output rises quickly to a maximum upon firing, then decays back to black along an exponential curve.

Image

The standard engineering term for expressing flash duration is T.5. This specifies the time the intensity of the flash is above 50% of its maximum value. Because of the exponential discharge curve of light output, another 50% of light is emitted after the T.5 time period and can add considerable motion blur.

Because of this, the T.1 spec was introduced to give the user a better idea of the action stopping effectiveness of a flash unit. T.1 specifies the length of time the intensity is above 10% of maximum and is thus more closely relates to an equivalent camera shutter speed in terms of how well action can be stopped with flash.

>> Click Here for more info on Syncing to the Camera - Sync Speeds

If the T.1 time is not specified, the user can reliably expect it to be approximately 3 times as long as the T.5 spec. Thus, a unit simply specified as having a flash duration of 1/1000 second can assume this to be the T.5 spec, and can expect the T.1 performance and effective action stopping to be approximately 1/300 second.

Control of flashpower is accomplished in one or more of several ways.

Thyristor Control:
In battery operated on-camera style flashes, a “thyristor” circuit abruptly shuts off the flashtube at the right moment as indicated by the desired power level. This results in a sharp cutoff of light devoid of the characteristic trail-off of studio flash. Reductions of flashpower result in shorter and shorter flash durations.

Thyristor control is relatively impractical in studio flash units due to the high power levels involved. In order to implement thyristors in studio flash units, it is necessary to limit the full power flash duration to relatively long values due to power handling capabilities of available components. Even if it were practical to use thyristor control of studio flash, the full power characteristics would still exhibit the trail-off shown in the graph above.

There are a few thyristor controlled studio flash units on the market in the 150 to 300 Ws range. Their full power flash durations are typically around 1/150 second (T.5 method), which relates to effective action stopping speeds (T.1 method) on the order of 1/50 second. This, of course, defeats the purpose of thyristor control since the user is seeking shorter, not longer flash durations. In comparison, AlienBees and White Lightning units in this power range offer T.5 flash durations on the order of 1/3000 to 1/6000 second and T.1 flash durations between 1/1000 and 1/2000 second. Zeus units have an even faster flash duration at a given power level and are thus among the best flash systems available for sports and other uses that require high power levels at fast flash durations.

Variable Voltage Control:
The primary method of flashpower control in most monolights such as AlienBees, White Lightning and most competitive products is by varying the voltage to which the flash capacitors are charged. This allows for a relatively wide range of power control without compromising the full power flash duration.

However, this method causes flash durations to become longer, rather than shorter, as power is reduced. As power is reduced from full power to 1/32 power, the flash duration typically approaches being twice as long. There is also a slight change in color temperature of about 60° Kelvin per halving of power with this method.

>> Click Here for more info on Color Temperature / Color Balance

Capacitor Switching:
The third method of controlling flashpower is by switching flash capacitors in and out of the circuit. When half of the capacitors are switched out, the power halves, as does the flash duration, while the color temperature remains essentially constant. This method is widely used in separate power pack type systems for setting ratios, often in conjunction with variable voltage control for fine-tuning.

The White Lightning X-Series X1600 and X3200 monolights and the Zeus power packs use both capacitor switching and variable voltage means to achieve a wide range of control of power and flash duration. For example, the X1600 (when set normally) provides maximum 660 Ws of power at a 1/1600 second T.5 flash duration. Using the variable power slider, the power may be reduced to as low as 20 Ws and the flash duration will then lengthen to about 1/1000 second. Switching the unit to the quarter power position reduces the maximum power to 165 Ws at 1/6400 second T.5 flash duration. Using the variable power slider in quarter power mode, the power may be reduced to as low as 5 Ws and the flash duration will lengthen to about 1/3600 second. The X3200 mirrors these parameters, but at twice the flashpower and with flash durations twice as long.

Multiple Heads:
When multiple flashheads are connected to a power pack system such as the Zeus, the flash durations become shorter as more heads are connected. For example, one head on a Zeus 1250 at full power (1250 Ws) will yield a T.5 flash duration on the order of 1/2000 second, while connecting two heads will provide the same total light output (625 Ws from each head) at a T.5 flash duration around 1/4000 second.

Zeus power packs can also be set to deliver 1/4 power (via capacitor switching) to a single head. On the Zeus 1250 pack, this yields 312 Ws at a 1/8000 second T.5 flash duration. A Zeus 2500 pack in this mode delivers 625 Ws to a single head at a 1/4000 T.5 flash duration.

When two heads are connected but ratios are set to provide unequal power to the heads, unequal flash durations from the heads will result.


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