Flash Duration

The normal style of strobe flash (including all those listed here) varies light output by reducing the duration of the flash pulse. Flash duration is important for two reasons. First, a pulse longer than the time the shutter is completely open is partially wasted. The camera's shutter sync speed (¹⁄₁₈₀th of a second, for current Pentax dSLRs) is the absolute limit for this, but if the flash pulse can't keep up with even slower shutter speeds underexposure will result. Second, when using a flash as the primary light source, shutter speed is irrelevant for purposes of freezing motion, and only flash duration matters.

A flash pulse is a curve, not a straight on-off thing. This makes measuring flash duration tricky, and so it's done in a tricky way. The standard normally used is the T.5 time, which is the time in which the light output is at least half of the brightest level. This number isn't particularly helpful for any practical use, but can serve as a standard for comparing between different models.

Another measurement is the T.1 time: time in which the light output is above 10% of the peak. This comes much closer to representing the meaningful duration of light output. A rule of thumb is that T.1 duration is approximately three times the T.5 time, but there's no absolute correlation.

Pentax, Sigma, Promaster, Tumax

Pentax provides no duration information for the AF200FG, but for lists the T.5 time for both the AF360FGZ and the AF540FGZ as approximately ¹⁄₁₂₀₀ seconds at full power and ¹⁄₂₀,₀₀₀ seconds at minimum power. Unfortunately, Pentax customer service was unable to provide more detailed information on request. It's a bit odd that the same numbers are repeated for both flashes, since not only does full power vary greatly between them but also minimum power is ¹⁄₃₂ of full in one case and ¹⁄₆₄ in the other. We can estimate that the T.1 time is about ¹⁄₄₀₀ seconds at full power and that stopping-power goes as short as ¹⁄₆₀₀₀ with less-powerful flashes, but it doesn't seem safe to extrapolate too much from these numbers.

Sigma simply states that the flash duration for full-power firing is about ¹⁄₇₀₀ seconds. No details are provided, but it seems safe to assume that this is the standard T.5 measurement, which would put the T.1 speed at around ¹/₂₃₃ seconds — give or take. My request for more details was declined — "I spoke with our corporate office regarding this issue and unfortunately we cannot provide you with privileged information to you as it is not available at this time. We apologize for the inconvenience."

Promaster claims a duration range of ¹⁄₁₀₀₀ to ¹⁄₃₀,₀₀₀ seconds, and Tumax says ¹⁄₁₀₀₀ to ¹⁄₂₀,₀₀₀ seconds. Again it seems unwise to extrapolate much from this beyond the guess that this refers to T.5 time.

There are reports that the Tumax DPT3 series underexposes when a full charge is needed; this may be a symptom of a too-long flash duration at maximum power.

Metz — Useful Data!

Metz does not provide T.5 times at all, but instead gives full tables of T.1 durations at each power level stop. The tables are not identified as such, but Metz customer service confirms that this is the measurement method used. This is excellent news, since T.1 is the actually-useful number. The ranges given are as follows:

  • 44 AF-1: from ¹⁄₁₂₅ seconds at full power to ¹⁄₁₅,₀₀₀ seconds at ¹⁄₆₄ power
  • 48 AF-1: from ¹⁄₁₂₅ seconds at full power to ¹⁄₂₅,₀₀₀ seconds at ¹⁄₁₂₈ power
  • 50 AF-1: from ¹⁄₁₂₅ seconds at full power to ¹⁄₂₅,₀₀₀ seconds at ¹⁄₁₂₈ power
  • 52 AF-1: from ¹⁄₁₂₅ seconds at full power to ¹⁄₂₅,₀₀₀ seconds at ¹⁄₁₂₈ power
  • 54 MZ-4i: from ¹⁄₂₀₀ seconds at full power to ¹⁄₂₆,₀₀₀ seconds at ¹⁄₂₅₆ power
  • 58 AF-2: from ¹⁄₁₂₅ seconds at full power to ¹⁄₃₃,₀₀₀ seconds at ¹⁄₂₅₆ power

The reduced-power numbers are significantly faster than anything available from the other manufacturers. This is partly because Metz's flashes allow several stops more latitude in reducing flash power than the competition.

At the other end of the scale, though, there is a concern that the flash duration of the AF models at full power exceeds the ¹⁄₁₈₀ second sync speed. Metz technical support confirms that when full flash is necessary, the shutter speed must be kept below the listed duration. However, it should be noted that Metz is the only maker to give solid numbers on this at all, so I would be careful about holding this against them unless we can get better real data from anyone else. Also, when the flash is used as the primary light source, my K10D in P mode prefers to set the shutter speed to ¹⁄₆₀ with any P-TTL flash, well within the safe range.

Metz does not provide a detailed chart of flash duration for the 36 AF-4 and other more-basic flashes, probably because without manual control that information would be of little use. The duration is listed as ¹⁄₅₀₀ to ¹⁄₃₀,₀₀₀ seconds, and Metz technical support confirms that this is indeed the T.1 number — the tech even tested in the lab to double check at my request. (The T.5 duration is about ¹⁄₁₀₀₀.)

Semi-Practical Duration Chart

The T.1 timing data can be used in combination with guide number information to calculate the maximum distance at which one can freeze motion to a given duration. The following chart is for ISO 100 at f/2.8. Multiply the distance by √2 in order to increase ISO or aperture by one stop or conversely divide to go the other direction. It assumes maximum zoom.

Again, please remember that this is based on manufacturer-published data rather than on actual measurements. There's really not going to be any useful way to calculate this without careful hands-on testing and an oscilloscope.


The ✖ indicates a very rough approximation of the range and duration of the Pentax AF540FGZ at minimum power based on the given T.5 number. (The AF360FGZ estimate is about a tenth of a meter less, and therefore basically in the same approximate range.) This is really just an educated guess, and I only include it at all because Pentax-branded equipment is obviously of particular interest.

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