Discussions (50,730)

Measuring Flash Duration

view profile

Maurice Ribble says:

I made a video showing how to measure flash duration using a light sensor and an oscilloscope. I explained the advantages of xenon flash tubes and their downfall. I also go describe a trick modern speed-lite flashes use to get their short durations.

After all this I measure the flash duration of a Canon 580 EXII and a Yongnuo 460 flash. Much to my surprise the Yongnuo flash had less than half the flash duration. I was quite surprised by this and was wondering if anyone here has ever noticed something like this.

The video is at www.techphotoblog.com/tpb-23
11:07AM, 1 October 2011 PDT (permalink)

view photostream

☣ cUKi says:

Thanks, that's interesting. I've been thinking of doing this kind of tests myself (mostly as a reason of getting and playing with an oscilloscope!). The shape of the curve doesn't look familiar, maybe the sensor or the power level used is still affecting it somehow (the pulse might be trimmed just as it reaches its apex). I'm not very surprised YN flashes are faster than Canon's since they're a bit more recent. For me it doesn't really matter so much, I'd still trade my three YN flashes for three Canon 580 EXII ones :)
31 months ago (permalink)

view photostream

rudy__ says:

Some errors. It was stated that MOSFETS are used to switch off the power in speedlights. Actually IGBTs Insulated Gate Bipolar Transistors are used (and previously SCRs) not MOSFETS. While an erro not a significant one. (who cares)

Then it was stated that on studio strobes resistance is used to "weeken the effect of the flash". Nobody uses resistance in the discharge circuit to control light output. It is not practical. Most often the voltage on the photoflash capacitor is adjusted. Some units will also switch in different banks of capacitors to get more variation. And a few will use IGBT control just like in modern speedlights.

One concern that I have with the test is that it looks like the sensor ws driven into saturation and the resultant waveform looks distorted compared to what I have measured with a 580EX (not 580EX II).

While it is implied that a shorter flash duration would be more desirable in some cases that may not be true as the color temperture of the light output can vary more with shorter durations.
31 months ago (permalink)

view photostream

Victor W. says:

I have to agree with some of the points Rudy made, (the man knows his stuff) I also haven't seen a studio flash use resistance to vary the flash output since the old Bowns monolites of the 1960's and 70's! (Studio flashes being far superior nowadays in controlling flash duration and importantly constant colour output with some rather clever circuitry).

I also have to agree with what looks like sensor saturation as Rudy pointed out and can lead to inaccuracies in your preliminary findings.

Maybe your point about 'spark gap' flash units being the major follow on interest......

As for the differences between the Canon and Yongnuo, again your findings were quite preliminary and seemed to be more concerned with flash duration rather than flash output.
Try popping the Canon at a lower power with 1 second intervals measuring each flash output and you'll see the Canon microprocessor and IGBT circuitry will accomodate the loss in continuous operation by lengthening the flash duration to achieve relative constant output. The Yongnuo will not come anywhere near that in functionality and flash output will actually decrease despite your setting........there's usually a good reason for a much lower price ;-)

However, most of us mere mortals will be hardly concerned which is just as well, you'll have a real nightmare when you start looking at colour vs power output! (Which is where many modern studio flash units win hands down.)
31 months ago (permalink)

view photostream

Mr. Speedlight says:

Then it was stated that on studio strobes resistance is used to "weeken the effect of the flash". Nobody uses resistance in the discharge circuit to control light output.

A friend bought a Norman 800? anyway 800 WS and it used a dump through resisters. I fired too many pops at partial power not knowing this since my Norman P2000X uses capacitors that give 200 WS, that being it's minimum. The P2000X uses heavy duty switches to select 200, 400, 800, 1600 or 2000 WS. Anyway my friend's Norman power pack started smoking so I quit and it wasn't damaged. We took it to Norman for inspection. I think my friend should have returned the unit and had a custom made Norman P2000X built with 125 WS capacitors. That was a practice among portrait photographers at the time.

I'll certainly agree that resistance is impractical and I hope neither Norman nor any other maker uses it today.

Dave Hartman
31 months ago (permalink)

view photostream

Murray McMaster says:

Isn't the 460 1/2 the power of the 580II? & wouldn't that explain the durations?
31 months ago (permalink)

view photostream

PeteTsai says:

in case anyone is interested in hss durations I blogged about it a while ago..

petetek.blogspot.com/2010/09/high-speed-sync-output-tests...
31 months ago (permalink)

view photostream

Maurice Ribble says:

Awesome information folks. Thanks for the corrections. I have updated the show notes to include a link here for more information.

After some thought I'm pretty sure Rudy is correct about the IGBTs. I also admit that my studio knowledge is based on a very old article I read so the comment about modern studio flashes also probably has truth to it.

Most of my work deals with high speed photography and I'm willing to sacrifice flash power and light color quality for speed. One example is the air gap flash I build has a very pronounced red-ish tint. I've found this is common to most (possibly all) air-gap flashes, but it's something I can live with to get sub-microsecond flash durations. I'm just pointing out my perspective so others here understand it and I value the comments from people coming from other perspectives.

Does anyone here have a feeling for minimum duration on a xenon flash tube? I'm wondering if I should look into further lowering the power and duration of my yongnuo flash. If I could get it fast enough to mostly replace an airgap flash that would be really awesome.

Thanks for all the great information so far!
31 months ago (permalink)

view photostream

Alfredk says:

I doubt that you will be able to get any shorter duration than with an air gap unit, the xenon gas which makes conduction easier and bridge longer distances, also lengthens the duration when compared to an air gap.
Ionized air has almost zero resistance and therefore the discharge is extremely powerful and fast!
Originally posted 31 months ago. (permalink)
Alfredk edited this topic 31 months ago.

view photostream

rudy__ says:

Based on what I have read (i'm not into high speed photraphy so I have not done any experiments yet) Alfred is correct. From the work done by others they tend to use air gaps to get the time down. That or specialize flash units not like our speedlights.
31 months ago (permalink)

view photostream

Maurice Ribble says:

Sorry, I must not have been clear. I highly doubt you could get down to the duration of a air gap flash (0.5 us) with a xenon tube, but if you could get close enough to freeze projectiles (1-2 us) that would allow a lot more people to photograph projectiles. There are some serious disadvantages to air gap flashes. Such as 25K volts, very expensive flashes, or expensive components if making your own. Just the capacitor for air-gap flashes tends to be $500-$1K if you buy it new.
31 months ago (permalink)

view photostream

alan_sailer says:

Maurice,

I just saw your video on measuring flash duration.

I can't dispute your conclusion, since I have not done any measurements on a Yongnuo.

But using the SD5600 optoschmidt detector to measure flash duration raises a few alarm bells for me.

The SD5600 is not an analog detector. It has a built in amplifier and a Schmidt detector output, so the output waveform is a digital representation of a highly analog event.

One commenter said that the waveforms look clipped, this is why.

I am also concerned that the waveform you show on the scope has a rounded leading edge. The SD5600 is wicked fast, in the ten nanosecond range, so seeing rounded pulses is not right. I don't believe that your projectile sensor circuit board uses a pull up resistor at the output of the SD5600, I seem to remember that this can cause that effect.

I have measured the waveform of a pellet moving at over 800fps passing in front of an SD5600. The pulse was a near perfect rectangle, with a duration of about 25 microseconds. So I know it can put out a clean pulse.

The pull-up resistor should be about 300 ohms.

I don't know a lot about commercial xenon flashes, but I believe that they use two different ways of specifying duration. One is to measure (analog) the light output at half power, the other uses 1/10 power to give an idea if there is a long flash tail.

T.5 and T.1 are the terms I have seen.

I don't believe that you method is measuring either of these, although I am not really sure what you are really measuring.

Cheers.

P.S. I also don't believe that your air gap flash has a red color. The afterglow is red, but the main flash is so blue that it is darn near ultraviolet.
31 months ago (permalink)

view photostream

Andy Gock says:

I thought some of you guys might be interested. Last night I did some flash duration measurements on the Canon 580EX and Nikon SB24, SB26, SB28 and SB80DX. All t.1 times.

www.gock.net/2012/01/flash-durations-small-strobes/

Other than the results, I also included diagrams and circuits on how i measured it.
27 months ago (permalink)

Would you like to comment?

Sign up for a free account, or sign in (if you're already a member).