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FORUMS General Gear Talk Flash and Studio Lighting 
Thread started 03 Jan 2008 (Thursday) 21:44
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The amazing speed of optical slaves

 
Curtis ­ N
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Jan 03, 2008 21:44 |  #1

Well they might just give me back my "King Measurebator" title for this (not that I want it).

After dealing with a sync speed issue with my Elinchrom Skyports, I began to wonder how much an optical slave might delay the flash it's attached to and make such an issue worse. So I decided to test it.

This shot shows my ceiling being illuminated by two flash units - The one on the left is triggered by the Skyport radio set and the one on the right is attached to an optical slave. So in this setup, the flash on the left is the "master".

IMAGE: http://performancephoto.smugmug.com/photos/239389454-O.jpg

When the shutter speed is increased beyond the camera's X-sync speed, the flash fires while the second shutter curtain is closing. The dark area in the bottom of the frame is the part of the sensor shaded by the shutter curtain.

IMAGE: http://performancephoto.smugmug.com/photos/239389463-O.jpg

Notice that the height of the black area is nearly the same on both sides of the frame. This indicates that the flash connected to the optical slave fired almost simultaneously, even though it didn't fire until it "saw" the other flash. If the second flash fired with any significant delay, the black bar would be higher on that side of the image.

IMAGE: http://performancephoto.smugmug.com/photos/239389488-O.jpg

The difference between the last two images represents the distance the shutter curtain travelled in 0.00625 seconds (1/1600 second). The nearly imperceptible difference in the height of the black area on either side of the frame indicates that the delay of the optical slave was less than 1/10,000 second.

What does all this mean? Well, it means that an optical slave will not cause a reduction in the practical maximum shutter speed with flash, even if you're using a camera with an electronic shutter that can sync at 1/1000.

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Welby
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Jan 03, 2008 21:47 |  #2

Another handy post thanks Curtis ;)


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PacAce
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Jan 03, 2008 21:49 |  #3

No surprise there, Curtis, since the optical slave is working at the speed of light and there's not much to the circuitry--just a photo-sensitive transistor and an SCR semiconductor switch or equivalent, both also working at the speed of light. :)

On the other hand, a digital triggering circuit needs to convert the "fire" signal into digital code which is transmitted in series by the Tx and then received by the Rx, decoded and then converted to a signal that triggers the triggering circuit. That serial conversion before transmission and then after reception is what's slowing the RF triggering devices. Actually, the same applies to the ETTL IR signals, too. :)


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Curtis ­ N
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Jan 03, 2008 21:59 |  #4

Maybe no surprise to you smart people, but for those of us whose entire knowledge of electronics could fit on a postage stamp, it's pretty darned amazing. :cool:


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PacAce
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Jan 03, 2008 22:07 |  #5

Curtis N wrote in post #4626256 (external link)
Maybe no surprise to you smart people, but for those of us whose entire knowledge of electronics could fit on a postage stamp, it's pretty darned amazing. :cool:

Have you seen the amount of electronics they can put in a postage stamp these days? Half a century ago, the equivalent amount of electronics would have required a big room with a connecting room to accommodate the air conditioning system required to cool the electronics. Now, that's pretty amazing. :D


...Leo

  
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bieber
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Jan 03, 2008 22:07 |  #6

The current front-page article on Strobist, I believe, is about a guy who found that the only way he could get remote Speedlights to keep up with his 1DII was to use optical slaves; ETTL and radio triggering didn't work fast enough...


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PacAce
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Jan 03, 2008 22:15 |  #7

bieber wrote in post #4626289 (external link)
The current front-page article on Strobist, I believe, is about a guy who found that the only way he could get remote Speedlights to keep up with his 1DII was to use optical slaves; ETTL and radio triggering didn't work fast enough...

I'm not sure I quite understand that. The flash sync speed is only 1/250. The PW units, for example, can keep up with that shutter speed without a problem so I'm curious to know in what way the RF triggering units failed to work. Was he referring to the frame burst rate?


...Leo

  
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Jan 03, 2008 22:19 |  #8

I have said this before, but optical slaves are my primary mode of triggering all of my flash units except one. That "one" typically has a long cord, or a radio slave.

I only use something else when the use of the optical slave just does not work in that application. But, since I mostly do studio, or ad hoc studio work, I seldom need to use anything other that optical slaves.

In addition to being fast, they are:

  • very inexpensive
  • require no batteries
  • typically do not wear out (I have some that I still use that are decades old)
  • Are almost indestructible (I mostly have the units that are the simple circuits totally encased in resin). You could drive a car over them and they would typically still be fine.
  • Did I mention they do not require batteries.

Enjoy! Lon

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Curtis ­ N
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Jan 03, 2008 22:19 |  #9

Regarding the Strobist post  (external link)bieber mentioned, It wasn't a sync speed issue, it was a frames per second issue. He said he tried his "Bogen radio slaves" and they didn't pop on every frame at 9 fps.


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DocFrankenstein
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Jan 03, 2008 22:26 |  #10

FlashZebra wrote in post #4626382 (external link)
I have said this before, but optical slaves are my primary mode of triggering all of my flash units except one. That "one" typically has a long cord, or a radio slave.

I only use something else when the use of the optical slave just does not work in that application. But, since I mostly do studio, or ad hoc studio work, I seldom need to use anything other that optical slaves.

In addition to being fast, they are:
  • very inexpensive
  • require no batteries
  • typically do not wear out (I have some that I still use that are decades old)
  • Are almost indestructible (I mostly have the units that are the simple circuits totally encased in resin). You could drive a car over them and they would typically still be fine.
  • Did I mention they do not require batteries.
Enjoy! Lon

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PacAce
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Jan 03, 2008 22:58 |  #11

Curtis N wrote in post #4626384 (external link)
Regarding the Strobist post  (external link)bieber mentioned, It wasn't a sync speed issue, it was a frames per second issue. He said he tried his "Bogen radio slaves" and they didn't pop on every frame at 9 fps.

I couldn't get my flash mounted on the camera (1Dmk3) and set to power level 1/128 to pop on the 9th frame either at the highest burst rate. For some reason, the flash always stopped firing after the 8th frame. Tomorrow, I'm going to use an external power source and see if I can get the number of frames passed 8.

I did try with the PW units and at 10 fps, the flash fired every other frame although every once in a while, it did fire two in a row. I have a feeling that if I had two MultiMax units, I wouldn't have a problem firing the strobes at 10 fps because the MM can be set to 30 fps. Unfortunately, the PW Plus units can't go that fast. With the Plus units, the flash fired once every 2 or 3 frames.


...Leo

  
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Lotto
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Jan 04, 2008 04:11 as a reply to  @ PacAce's post |  #12

More flash pron for the King Measurebator :)

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mbellot
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Jan 04, 2008 05:47 |  #13

PacAce wrote in post #4626605 (external link)
I couldn't get my flash mounted on the camera (1Dmk3) and set to power level 1/128 to pop on the 9th frame either at the highest burst rate. For some reason, the flash always stopped firing after the 8th frame. Tomorrow, I'm going to use an external power source and see if I can get the number of frames passed 8.

Probably just simply that the flash recharge couldn't keep up.

1/128 is 2^7 or 8 "stops" (2^0 = 1 or full power) difference in power level. During the 9 shot burst the flash would have had to manage to recharge at least 1/8 power (possibly more) to actually have sufficient juice for the 9th frame.




  
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Jan 04, 2008 06:01 |  #14

Curtis N wrote in post #4626256 (external link)
Maybe no surprise to you smart people, but for those of us whose entire knowledge of electronics could fit on a postage stamp, it's pretty darned amazing. :cool:

lol... thanks Curtis..

.......... and now we know, the rest of the story.

I wish Canon would use something like the Nikons do for wirless from the camera.


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pjtemplin
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Jan 04, 2008 06:14 |  #15
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1/128 is 2^7 or 8 "stops" (2^0 = 1 or full power) difference in power level. During the 9 shot burst the flash would have had to manage to recharge at least 1/8 power (possibly more) to actually have sufficient juice for the 9th frame.

Huh? 1/128 is 8 stops difference, but it's 1/128th nonetheless. After 8 shots, it would have fired 8/128ths, or 1/16th of full power. I just grabbed a 580EX II out of my bag, set it for multi 1/128th, and with a flash rate of 8hz it's good for 80 sequential pops. At 16hz it's good for 50, and at 20hz or higher it's good for 40.


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The amazing speed of optical slaves
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