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09-14-2018, 06:34 PM   #1
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Flash Duration t0.1, t0.5

https://profoto.azureedge.net/cdn/04a5f25/globalassets/inriver/resources/pro...r-guide_en.pdf

If you go to page 32 of these specs it lists the flash durations and power output.

What does t0.1 and t0.5 mean?

And what does watt output represent? or how is it translated into Guide number?

Thanks

09-14-2018, 07:13 PM - 1 Like   #2
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QuoteOriginally posted by derekkite Quote
What does t0.1 and t0.5 mean?
This article from strobist does a good job of explaining it





In a nutshell T0.5 and T0.1 are measurements of time it takes for the flash pulse to drop to 50% [T0.5] of its light and T0.1 shows how long the flash pulse takes to drop to 10% of its peak light output. A flash with a faster T0.1 speed will be better at stopping motion. For example the Elinchrom BX 500Ri is pretty typical of flashes in the 500-600 w-s class, its 1/1558 sec t.5 time translates to 1/519 sec t.1: so it stops motion just like a 1/500 sec shutterspeed. You typically will get shorter durations by going to a flash in the 250-300 w-s class. The smaller capacitors drain quicker. BX 250Ri has a t.5 of 1/2762, which makes the t.1about 1/920 sec.

Last edited by Digitalis; 09-14-2018 at 07:41 PM.
09-14-2018, 07:14 PM - 1 Like   #3
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t0.1 is the time between when the power level of the flash light value is between 0.1 of the max - from rise to fall of the light curve. Similarly, t0.5 is the time between the half power levels (0.5!) of the peak. This gives you some idea of how long the flash light burst lasts.

In my opinion, these are only somewhat useful numbers. What you really want is the fraction of time that some fraction of the output light power lasts - that's what lights up your subject, and a big deal if things are moving/changing during the flash illumination. I prefer what I call p50 or p90 - the time during which 50% or 90% of the total light output power takes place. These are more difficult to measure than t0.1 and t0.5 - as well as typically less "nice" for the flash manufacturers (i.e. they are LONGER than the p values).

See my measurements for the AFG540FGZ for some real-world results and comments: Flash Duration Measurements - AF-540FGZ - PentaxForums.com

Last edited by AstroDave; 09-14-2018 at 09:35 PM. Reason: change "t" to "p" at end of middle paragraph
09-14-2018, 07:40 PM   #4
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QuoteOriginally posted by Digitalis Quote
This article from strobist does a good job of explaining it





In a nutshell T0.5 and t0.1 are measurements of time it takes for the flash pulse to output 50% [t0.5] of its light and T0.1 shows how long the flash pulse takes to drop to 10% [t0.1] of its peak light output. A flash with a faster t0.1 speed will be better at stopping motion. For example the Elinchrom BX 500Ri is pretty typical of flashes in the 500-600 w-s class, its 1/1558 sec t.5 time translates to 1/519 sec t.1: so it stops motion just like a 1/500 sec shutterspeed. You typically will get shorter durations by going to a flash in the 250-300 w-s class. The smaller capacitors drain quicker. BX 250Ri has a t.5 of 1/2762, which makes the t.1about 1/920 sec.
Thanks. That makes sense. You can then figure where on that curve the light emission drops below the level which exposes.

I'm trying to freeze movement at about 1/4000. Speedlights do that at about 1/4 or 1/8 power. I had 5 of them firing at once which worked ok but was rather complicated to set up. I would like to replace that with a battery powered light, or two.

Now to figure out how watts output translates into guide numbers or some idea of exposure.

09-14-2018, 07:53 PM   #5
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QuoteOriginally posted by derekkite Quote
I'm trying to freeze movement at about 1/4000. Speedlights do that at about 1/4 or 1/8 power. I had 5 of them firing at once which worked ok but was rather complicated to set up. I would like to replace that with a battery powered light, or two.



if you want durations as short as 1/4000th, there are two ways to go about this: and it boils down to IGBT flash units Vs variable voltage flash units. With IGBT flash units, the pulse of light get shorter at lower power settings*. Variable voltage flash units work the opposite way: the higher the power setting, the faster the pulse gets.

From reading the specs the Profoto flash fits the profile of an IGBT - however this might not be what you want, buying a 250ws flash only to use it at its lowest power setting seems a bit daft to me - especially when you are using it to replace multiple flash units. Getting a mains powered Variable voltage flash head might be better for you as the short duration at full power gives you much more light to work with.


* and at full power the flash pulse can be really slow, I measured one popular IGBT flash at T0.1 of 1/125th @ 1:1 power

---------- Post added 2018-09-15 at 01:54 PM ----------

QuoteOriginally posted by AstroDave Quote
These are more difficult to measure than t0.1 and t0.5
All you need is an oscilloscope, photocell, a series resistor in the 5-10k range, a 1uF cap, a little experience with soldering and it wouldn't be hard to rig up a way to empirically measure the rise time.

Last edited by Digitalis; 09-14-2018 at 08:29 PM.
09-14-2018, 09:18 PM   #6
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Ah. I see that Paul C Buff has a couple that work like that. The Godox one seems to have a very short duration at high power.

Flash Duration Analysis with the Sekonic L858D-U ? The Broketographers

has some timings.

Thanks for the information. Knowing what to look for really helps.

Having a dish that aims the light will increase the light as well.
09-14-2018, 09:30 PM   #7
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QuoteOriginally posted by Digitalis Quote
All you need is an oscilloscope, photocell, a series resistor in the 5-10k range, a 1uF cap, a little experience with soldering and it wouldn't be hard to rig up a way to empirically measure the rise time.
It's not the rise time, it's the area under the light curve - you need to integrate the light curve to get the "power" values. Nobody but me seems to have ever done that. As you say, it's not at all difficult, but you do need a computer program of some sorts along with your scope and other equipment. (Which should be at least a phototransistor (microsecond response time), and ideally a photo diode (nanosecond response time) - a "photocell" (which I take to be an old style selenium, say - millisecond response time) is nowhere near fast enough for this!) You need the individual data points in the light curve. A modern digital storage oscilloscope provides this.

Oh, and no 1 uF cap! Time constant (rc) with a 10k resistor is 0.01 second!
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