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Archive 2013 · Wireless - setting off flashes simultaneously?
  
 
gladivs
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p.1 #1 · p.1 #1 · Wireless - setting off flashes simultaneously?


With the incredibly short flash durations with speedlight type flashes, I was wondering if there was any easy way to determine if a couple of flashes are really being set of simultaneously.

I guess if two identical flashes, set at the same power level, go off at the same time, you have twice as much power. But the flash duration is so short - say 1/1000 of a second - if there is the tiniest delay you wouldn't get double the power.

The reason I'm asking is because I had a situation last night where it appeared to me - just visually, watching the flashes go off - that a 580EX II and a Yongnuo 568EX were not going off at the same time.

What I think I can do is to measure the power of each with a flash meter, and then measure the power of the two going off at the same time. The question is, how do I add the results from the two individual tests to predict what the combined total should be?




Edited on Jul 23, 2013 at 11:44 PM · View previous versions



Jul 22, 2013 at 12:44 AM
scottam10
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p.1 #2 · p.1 #2 · Wireless - setting off flashes simultaneously?


The shutter is open for a much longer period than the flash duration (eg 1/160th shutter speed Vs 1/1000 flash duration). As long as the flashes both go off while the shutter is open, the flashes will both contribute to the exposure, though they may not 'freeze' motion as well as a single flash

- are you seeing a double exposure effect on moving subjects?

The easiest way to test this is to set all flashes to manual power (ie not ETTL), set camera to manual with shutter speed below the X-SYNC speed for your flashes, and take shots with a single flash firing and with both flashes firing. The photo with both flashes should be much brighter.
- if you have trouble, try reducing the shutter speed to give both flashes a chance to get into the exposure - make a note of this shutter speed and stay below it when using 2 flashes together

This may be a 'penalty' of off-brand flashes, perhaps 2 Canon flashes would fire together better.
And if you need a single 'flash', do you actually need 2 flashes - can you reduce the flash power required by opening up your aperture or raising your iso so you're not working the flashes as hard and allow the job to be done by a single flash unit?



Jul 22, 2013 at 01:38 AM
BrianO
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p.1 #3 · p.1 #3 · Wireless - setting off flashes simultaneously?


gladivs wrote:
...I guess if two identical flashes, set at the same power level, go off at the same time, you have twice as much power. But the flash duration is so short - say 1/1000 of a second - if there is the tiniest delay you wouldn't get double the power.


As Scott said, since the sync speed of Canon cameras is 1/250 or less, even if the two flashes going off were separated by a few milliseconds they would both contribute to the exposure, so yes, you'd get double the exposure value.

I've fired as many as five Speedlites together, and never had any trouble, and I know of one photographer who has ganged 8 onto a light stick for shooting surfers and motorcyclists.



Jul 22, 2013 at 05:49 AM
alohadave
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p.1 #4 · p.1 #4 · Wireless - setting off flashes simultaneously?


gladivs wrote:
The reason I'm asking is because I had a situation last night where it appeared to me - just visually, watching the flashes go off - that a 580EX II and a Yongnuo 568EX were not going off at the same time.


First of all, how are you triggering? ETTL for the 580EX and optical for the YN? If so, you are seeing the pre flash of your Canon flash going off before both flashes fire.

If you are seeing both flashes in your exposures, then they are both firing in time, as explained by others here. Unless you are shooting really fast stuff like water drops or bullets, the timing really doesn't matter that much.




Jul 22, 2013 at 12:13 PM
BrianO
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p.1 #5 · p.1 #5 · Wireless - setting off flashes simultaneously?


alohadave wrote:
...ETTL for the 580EX...


Why would someone meter an ETTL flash? The power level is set by the camera based on a through-the-lens reading, and so would change if the aperture were changed based on a meter reading.



Jul 22, 2013 at 06:27 PM
dmward
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p.1 #6 · p.1 #6 · Wireless - setting off flashes simultaneously?


If you saw preceptively different flash timing between the two flashes you were seeing pre-flash on one and exposure flash on the other. I doubt that any human has eyes good enough to see a mili-second difference between the start times of two flashes that are probably bright enough to cause momentary white out if shot directly at the retina.

And, as also stated, unless you are seeing obvious under exposure both are firing to contribute to the image.



Jul 22, 2013 at 09:14 PM
gladivs
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p.1 #7 · p.1 #7 · Wireless - setting off flashes simultaneously?


Thanks everyone. I probably should have mentioned I am concerned about freezing action.

I'm shooting in a fairly well lit venue, but the the lighting is very uneven with hot spots and dark spots. Because it is well lit, I keep my ISO relatively low (about 800-1000) and am shooting around F4. The ISO gives me a couple of stops difference between ambient and flash. I shoot a Canon 580EX at half power. I would love to use a smaller aperture, and I'd rather not push the 580 much above half power both for re-cycling as well as the longer flash duration.

I was trying a Yongnuo YN-568EX, which is a TTL version of their flash. I didn't like the fact that I could "see" a delay so I did not use it as part of the shoot this day.

The pre-flash thought had occurred to me, as I was experimenting with ETTL at the time. Due to the flashes using gobs of power on ETTL, though, I went back to setting the power manually.

I was hoping to do something more concrete in terms of proving one way or another if they are really shooting at the same time, and you've given me some ideas so thanks!




Jul 22, 2013 at 11:58 PM
 

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BrianO
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p.1 #8 · p.1 #8 · Wireless - setting off flashes simultaneously?


gladivs wrote:
...The pre-flash thought had occurred to me, as I was experimenting with ETTL at the time. Due to the flashes using gobs of power on ETTL, though, I went back to setting the power manually.


Even in Manual mode, if you're using the Canon optical Wireless Mode as opposed to radio triggers, there will still be command and control preflashes.

gladivs wrote:
...I keep my ISO relatively low (about 800-1000)


This still causes me pause, not because it's wrong, but because it's so, well, weird to my way of thinking. I cut my photographic teeth back when ASA 25 and ASA 100 films were normal, and ASA 160 and ASA 400 were considered fast.


Edited on Jul 23, 2013 at 12:13 AM · View previous versions



Jul 23, 2013 at 12:08 AM
gladivs
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p.1 #9 · p.1 #9 · Wireless - setting off flashes simultaneously?


Hmm, that's not something I would have expected. What information would the camera gather from a preflash? I can understand in ETTL, as it is determining how much power to allocate to each flash group.

One more thing to experiment with! Thanks



Jul 23, 2013 at 12:12 AM
BrianO
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p.1 #10 · p.1 #10 · Wireless - setting off flashes simultaneously?


gladivs wrote:
Hmm, that's not something I would have expected. What information would the camera gather from a preflash?


It's not the camera, it's the Slave flash. The Master is telling it to get ready to fire at the set power level and so on. The reason a Slave flash won't fire when another photographer's flash goes off nearby is that, unlike simple optical triggers that can be fooled, the Canon Wireless System (and Nikon's CLS, etc.) uses coded preflashes to handle the remotes.



Jul 23, 2013 at 12:17 AM
gladivs
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p.1 #11 · p.1 #11 · Wireless - setting off flashes simultaneously?


ah right. I forgot that.


Jul 23, 2013 at 12:18 AM
alohadave
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p.1 #12 · p.1 #12 · Wireless - setting off flashes simultaneously?


BrianO wrote:
Why would someone meter an ETTL flash? The power level is set by the camera based on a through-the-lens reading, and so would change if the aperture were changed based on a meter reading.


Who's talking about metering? I didn't mention it.



Jul 24, 2013 at 12:53 PM
BrianO
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p.1 #13 · p.1 #13 · Wireless - setting off flashes simultaneously?


alohadave wrote:
Who's talking about metering?


Did you read the very first post in this thread?

"What I think I can do is to measure the power of each with a flash meter, and then measure the power of the two going off at the same time. The question is, how do I add the results from the two individual tests to predict what the combined total should be? "



Jul 24, 2013 at 04:25 PM
alohadave
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p.1 #14 · p.1 #14 · Wireless - setting off flashes simultaneously?


BrianO wrote:
Did you read the very first post in this thread?

"What I think I can do is to measure the power of each with a flash meter, and then measure the power of the two going off at the same time. The question is, how do I add the results from the two individual tests to predict what the combined total should be? "


Okay. That's not what I was addressing. It has nothing to do with my comment and question.



Jul 25, 2013 at 03:06 AM





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