The Type of Lights Can Make a Difference
Over the last few decades, flash photography has changed considerably. New designs, more features, and flashy colors have flooded the market with an ever increasing amount of choices. Yet, most of these flashes can be divided into two basic groups: small less powerful Speedlights, typically designed to mount on top for your camera and more powerful, bulkier Studio Strobes designed for higher quantity and quality of light. Understanding the differences between the workings of these two categories is a fundamental step in understanding their application when performing high speed photography.
Studio strobes, (i.e. Alien Bees. White Lightnings, Speedatrons, Photogenics, etc. ) produce a flash by powering up a large capacitor and then dumping the accumulated power directly to the flash tube. Since the flash tube is directly connected to the large capacitor, the flash output jumps quickly to maximum output, but then slowly trails off as the capacitor discharges its entire charge. Only after fully discharging the capacitor will the flash then re-initialize the charging phase and allow another flash. If the light output of the Studio Flash at full power is plotted versus time, the relationship of discharge time to output level can be seen as shown in fig. 1
Figure 1 – Studio flash flash output curve
In order to vary the quantity of light, these Studio Strobe units vary the amount of total power provided to the capacitor before discharging to the flash tube. This method of flash provides a higher quantity of light that is better color balanced. Thus, these units produce an overall higher quality of light than Speedlights. However, since the large capacitor must always fully discharge, recycle times are longer, flash durations are longer and larger amounts of power are consumed.
To further complicate matters, Studio Strobe flash durations are measured according to two main standardized methods, t.5 and t.1 – neither of which accurately describes the total flash duration of the Studio Strobe. Both measure the time it takes the flash to come to the maximum output for the selected power setting plus the amount of time needed to return to a percentage of that maximum as the flash intensity trails off . Therefore, the t.5 method measures the time needed to reach the maximum output for the selected power setting plus the time needed to return to a point equal to 50% of that peak (see fig. 1). Similarly, the t.1 method measures the time needed to reach the maximum output for the selected power setting plus the time needed to trail off to a point equal to 10% of that peak as shown in (see fig. 1). This practice leaves a portion of the flash duration unmeasured. The prevailing reasoning behind these methods of flash duration measurement is the concept that at some point the amount of light being provided by the trailing off flash no longer truly effects the photographic results. There is an ongoing argument over exactly when this point occurs. In any case, Table 1 shows three Alien Bee models and their corresponding t.5 and t.1 duration times for both full power (largest selectable power output) and 1/32 power (the smallest selectable power output). As Table 1 shows, flash durations for Alien Bees (And Studio Strobes in general) are shorter at higher power settings.
Note: Some of the newer more expensive Studio Strobe units actually switch to lower power capacitors when using low power settings in an effort to somewhat reduce the longer flash durations.
|Flash Unit||t.5 Flash Duration||t.1 Flash Duration|
|Full Power||1/32 Power||Full Power||1/32 Power|
|B400 (160Ws)||1/6000 second||1/3000 second||1/2000 second||1/1000 second|
|B800 (320Ws)||1/3300 second||1/1650 second||1/1100 second||1/555 second|
|B1600(640Ws)||1/18000 second||1/900 second||1/600 second||1/300 second|
Table 1 – Alien Bee flash duration times
Speedlights, on the other hand, utilize a small capacitor or a series of small capacitors to power the flash tube. Unlike the Studio Strobes, this is not a direct connection. Between the power and the flash tube is either a thyristor, or an insulated gate bipolar transistor (IGBT). Both are basically switches that turn off the power to the flash tube after a certain amount of power has been discharged. At full power, a Speedlight acts just like a Studio Strobe does. It charges up the capacitors, and then discharges them to the flash tube, quickly building to maximum output, then output slowly trails off. This however changes when lower power settings are selected. Consequently, the resulting flash output to time graph, is very different from that of a Studio Flash on any setting other than full power. Since the flash output of a Speedlight at less than full power is stopped by a switch, the long trailing off output of a Studio Strobe no longer occurs. (See Figure 2-9.) The benefits of this design, is a shorter flash duration, less overall power consumption, and smaller units. The downside is that the flash produces a lower quantity of light and is less color balanced, so light quality is inferior to that of a Studio Strobe. Most importantly, flash durations for Speedlights are not regulated by how much charge is applied to the capacitors. The capacitors are always fully charged. The amount of illumination provided is directly controlled by how long the flash burst is . Any remaining charge stored in the capacitors after cutoff by the switch is retained for later use. Therefore, for high speed applications, Speedlights are the correct choice, as they have much higher speeds. However, one must still keep in mind their relatively low light quantity.
Figure 2. 1/1 Output power
Figure 3. ½ Output power
Figure 4. ¼ Output Power
Figure 5. 1/8 Output Power (time/duration division changed)
Figure 6. 1/16 Output Power
Figure 7. 1/32 Output Power
Figure 8. 1/64 Output Power
Figure 9. 1/128 Output Power
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