Flash pulse duration

Chuck Westfall’s October Tech Tips is out, and he answers a question regarding the flash pulse duration of Canon’s speedlites. Canon doesn’t publish these numbers so there’s quite a bit of speculation, but he points out one thing that I didn’t know: at lower power levels, the xenon bulbs used in the flashes are not as efficient as they are at high power. Therefore, a flash firing at 1/16th isn’t necessarily firing 16 times faster than it does at full power, since it’s putting out less than 1/16th as much light per millisecond.

Without actual numbers or the tools to do some testing, it’s impossible to say how much it matters. It’s probably still firing faster than it would at 1/4 or 1/8, which is important in flash-based macro because your flash pulse duration is, effectively, your shutter speed.

The bit I find most interesting is where he says “I suspect that if you use a higher power setting on the 430EX to match the output of the 550EX, the flash durations from both Speedlites will end up being very similar if not identical.” I’ve been considering the 430EX II over the 270EX, on the assumption that they’d have similar pulse durations at full power, and therefore the 430EX II would be faster since I’d be using it at lower power levels. It may be that the 270EX at 1/2 or 1/4 power isn’t as much of a compromise in speed as I had thought; in terms of price and weight, it’s obviously the better choice for macro.

This may seem like nitpicking, but it does have real application for the small animal photography I like to do. On a warm, sunny day, it’s simply not possible to get a sharp image of a fast-moving insect without a flash pulse fast enough to freeze its motion. 1/250th of a second is a looong time in that shooting scenario.