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Archive 2012 · Speedlite Communication - How does it work?
  
 
Todd Moon
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p.1 #1 · Speedlite Communication - How does it work?


I watched a video by Syl Arena where he was discussing using multiple Canon speedlites. It was very informative. One point he made was that the master speedlite communicates to the slaves using pulses of visible light from the flashtube which encode timing and power instructions for each slave group. He then demonstrated this by disabling the master speedlite from firing, and taking a picture. The master still made a flash of light using its flashtube, but it did not show up in the photo.

This guy is a pro and has his own Speedliting book, so I consider him somewhat of an authority on the matter. However, the ST-E2 transmitter doesn't have a strobe and emits infrared light to communicate to the speedlites.

So, what's going on at a low level? Do flashtubes naturally emit infrared light along with visible light, so the speedlites simply use what's available rather than having a separate, unnecessary IR transmitter? Or are the receivers on the speedlites sensitive to visible light and IR, and the ST-E2 uses IR because it's cheaper, less distracting, and visible light is simply not needed?

Here's the video. It's worth watching if you want to learn about using multiple speedlites. http://www.youtube.com/watch?v=hk7IRg1ApGE

Here's his book: http://amzn.com/032171105X

Edited on Dec 10, 2012 at 04:52 PM · View previous versions



Dec 10, 2012 at 04:43 PM
Will Patterson
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p.1 #2 · Speedlite Communication - How does it work?


The pre-flash is for exposure metering. All settings data is transmitted via infrared.


Dec 10, 2012 at 04:48 PM
Todd Moon
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p.1 #3 · Speedlite Communication - How does it work?


Will Patterson wrote:
The pre-flash is for exposure metering. All settings data is transmitted via infrared.


Why would the master need to fire for exposure metering when it's disabled? Its light would throw off the metering since it's not going to fire for the actual photo. Yet Syl clearly demonstrated that it fires a preflash even when it's disabled.



Dec 10, 2012 at 04:51 PM
Ian.Dobinson
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p.1 #4 · Speedlite Communication - How does it work?


The St-e2 DOES have a strobe . It just that its behind a red lens .
If you we're to remove the red lens you would see it flash just like a normal speedlight.




Dec 10, 2012 at 05:06 PM
Ian.Dobinson
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p.1 #5 · Speedlite Communication - How does it work?


Will Patterson wrote:
The pre-flash is for exposure metering. All settings data is transmitted via infrared.


Sorry Will but that's wrong. The master sends coded pre flashes out from the main tube. No mastering is emitted from the red lens part of a speedlight .
Just as if you use a 7D (or a couple of other bodies) the onboard flash has to be up to master.

It's also why a 580 works as a better master at longer distances or in brighter light. The coded flashes are not being filtered at the master.

To prove the point. Take a 580 master and cover the flash head . If you block out enough light then the slave will not fire.



Dec 10, 2012 at 05:14 PM
 

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Todd Moon
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p.1 #6 · Speedlite Communication - How does it work?


Ian.Dobinson wrote:
The St-e2 DOES have a strobe . It just that its behind a red lens .
If you we're to remove the red lens you would see it flash just like a normal speedlight.



Interesting. I was not aware of that. The red cover must be an IR filter, or at least a near-IR filter. Is that flash from the ST-E2 visible at all with the cover on? It must be that a portion of the light from a flashtube is IR. That's what gets through the filter to trigger the slaves, and is also the relevant part of what comes from a master speedlite when triggering slaves.

Edited on Dec 10, 2012 at 05:20 PM · View previous versions



Dec 10, 2012 at 05:15 PM
Ian.Dobinson
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p.1 #7 · Speedlite Communication - How does it work?


Todd Moon wrote:
Why would the master need to fire for exposure metering when it's disabled? Its light would throw off the metering since it's not going to fire for the actual photo. Yet Syl clearly demonstrated that it fires a preflash even when it's disabled.


There will be some form of metering going on as well. I don't think the slaves ever fire until the actual exposure . And as the current form of mettering takes place before exposure rather than being read off the film as it was previously the camera will have to have something to go on. But it can't know how far the slaves are from the subject etc , so you will always get a better result from a test fire.



Dec 10, 2012 at 05:19 PM
Todd Moon
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p.1 #8 · Speedlite Communication - How does it work?


Ian.Dobinson wrote:
There will be some form of metering going on as well. I don't think the slaves ever fire until the actual exposure . And as the current form of mettering takes place before exposure rather than being read off the film as it was previously the camera will have to have something to go on. But it can't know how far the slaves are from the subject etc , so you will always get a better result from a test fire.


The slaves must fire a metering preflash otherwise the system will have no idea how to account for them. The slaves ratio could be 1:1 where the master could be 1:128. The final photo will be lit almost entirely from slaves, so it has to prefire them so that the camera can calculate the final output to use. But if the master is set not to fire having it fire during the metering preflash would ruin the metering.



Dec 10, 2012 at 05:26 PM
Ian.Dobinson
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p.1 #9 · Speedlite Communication - How does it work?


Todd Moon wrote:
Interesting. I was not aware of that. The red cover must be an IR filter, or at least a near-IR filter. Is that flash from the ST-E2 visible at all with the cover on? It must be that a portion of the light from a flashtube is IR. That's what gets through the filter to trigger the slaves, and is also what comes from a master speedlite when triggering slaves.


Yeah the lens acts as the ir filter. but I think it's near IR rather than true IR . I've never noticed seeing the flash when I've seen an st-e2 in use , but maybe I've never been looking .

The slaves sensor is the almost matt plastic on the front of the flash rather than the shiny red plastic which is where the focus assist beam is. So if the sensor is covered or cannot see the master you will not get good master / slave



Dec 10, 2012 at 05:33 PM





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