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What Is A Watt Second?

Since almost all manufacturers test with 100 ISO film, and it is likely to assume that photographers understand the relationship between film speed and exposure, we will not discuss this further.

From/To 1 meter 2 meter 10 feet 1 meter n/a -2f/s -3f/s 2 meter +2f/s n/a -1f/s 10 feet +3f/s +1f/s n/a This chart provides a way of converting published distances for making a comparison.

The Inverse Square Law
The distance at which the units are tested is very important. Again, as there are no standards, some manufacturers use a distance of 1 meter. Others use a distance of 2 meters, while some use 10 feet. None of these distances are right or wrong, they're just different. Thanks to a property of light called "the inverse square law," it's easy to convert output from one distance to the other. Simply stated, when the distance is doubled or halved, the change in light will be 2 f/stops.

Much trickier is the conversion regarding the reflector. Manufacturers determine for themselves what angle their normal reflector will be. Some use a 35-degree angle, some use 50-degree, and one company uses a reflector that zooms, changing from 65- to 100-degrees. There may be a 4-stop difference between a 35- and a 100-degree reflector, even though the flash tube is producing the same amount of light. This makes a unit seem like it is more powerful than it really is. There is no standard mathematical formula for determining these conversions. Finishes in the reflectors will vary within a manufacturer's own line, not to mention from brand to brand, each being more or less efficient. So, this method of comparison is significantly less than perfect. While it may help choose which strobe systems should be considered, only a side-by-side comparison will be completely accurate.

To compare two units, set both on light stands, with flash tubes equidistant from the wall. Choose your standard distance — 1 meter, 2 meter, or 10 feet — it makes no difference, as long as the distance is exactly the same for each unit. I prefer the 2-meter distance as it is short enough to do in a small area, but long enough that a slight difference between the flash unit distances will not alter the results of the test significantly. Make sure the reflectors are the same angle and as close to the same finish as possible. Finally, with the flash units on full power, take individual meter readings of each, using the same meter, and compare the results.
While comparing flash units can be daunting, it's clear that the quality and range of accessories available in the marketplace afford photographers multiple options for creating their visions.