Flash Duration - meaning of the t.5 and t.1 measurement methods

WayneF

Senior Member
I posted this elsewhere, and it seemed to have some appeal, so I thought it ought to go here too. Second pass, added a bit.


t.5 and t.1 are common terms and methods for measuring flash duration.

Flashes have a rapid strong peak of light, and then slowly fades away (relatively slowly, a few milliseconds). It's a lingering decay, and it's hard to say when the duration reaches zero. So it is very hard to measure the duration (to say when it stops), but no one much cares when it finally reaches zero. Very low values have already lost effectiveness to much affect the exposure.

Engineers tend to measure difficult things at the half power points. So expediently (and precisely), they measure the flashes half power points (the time duration when decayed to half of peak value). The method is called t.5, which is easy to measure, but not very meaningful to photography exposures. But nevertheless, ISO flash specifications of duration are standardized at the half power points. There is of course more light left though, which does affect our photographs exposure. But the engineers are more concerned with an accurate measurement, meaningful to them.

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There are also t.1 measurements, when 90% of the light is seen, measured down to the 10% level. The flash decay is a standard RC curve, well studied, and t.1 is routinely considered to be 3 times longer than t.5. This is more meaningful to photography, however unless otherwise stated, flash specifications are the t.5 standard. It just always was... A couple of brands (Metz and Paul C. Buff) are promoting t.1 numbers, which are slower numbers, but more meaningful to photographers.

The above is true of most studio monolights, which implement lower flash power by lowering the voltage on the flash capacitor. So at less than full power, this makes them slower (maybe 2x longer at 1/32 power), and also shifts the white balance temperature lower (more red, cooler).

Speedlights are made different. The above is true of their Full power, but their lower powers are always started at full voltage and full power (after recycle), but implemented by then simply interrupting the current flow to the flash tube (thyristor type, IGBT chips today)... like a on-off switch, to stop it instantly. So then the light does not decay away, but is simply chopped off, instantly. This chop makes the duration be extremely fast at lower powers, hence the name speedlight, which is the standard way to freeze difficult motion. And since the cool tail is chopped off, they become more blue. Both effects are the opposite in (almost all) studio monolights. But there is no one value of flash color temperature, it varies with power level.

Here are the specifications for the Nikon SB-800 flash duration (in the specs section of the flash manual).

1/1050 sec. at M1/1 (full) output
1/1100 sec. at M1/2 output
1/2700 sec. at M1/4 output
1/5900 sec. at M1/8 output
1/10900 sec. at M1/16 output
1/17800 sec. at M1/32 output
1/32300 sec. at M1/64 output
1/41600 sec. at M1/128 output

Full power is still a regular NON-truncated tail, slowly decaying in the standard way. 1/1050 is the standard t.5 time. t.1 time would be 3x longer, or 1/350 second (and a more meaningful number, photograph exposure wise). FWIW (my notion), this must be why the cameras offering 1/320 second Auto FP say that it could reduce flash range, the 1/320 second shutter speed could cut off some of the weak 10% tail of a full power speedlight (lower power levels are very fast though, not affected).

But 1/2 power is different, truncated, chopped off. The two spec times are about the same number, only because Full power is t.5, measured at half power points. But actual 1/2 power is chopped off, and actually stops at the 1/1100 second. 1/2 power is about 3 times faster than full power. You just have to know how to interpret the numbers.

All the lower powers are chopped off, and thus are accurate duration measurements. So then there is no actual distinction between t.5 and t.1, the flash intensity simply just stops. 1/32 power and 1/64 power levels are extremely fast, and can stop hummingbird wings (but the flash has to be pretty close at low power, in a more dim ambient). This method is named "speedlight", because they can stop fast motion.

You might be interested in seeing Capability of flash units for high speed photography
 
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