Optical Slave Delay?

[Jamesino]

Suppose I'm using Canon's Speedlite optical slave system, the slave will fire when it detected that the master had fired right? But won't there be a delay? In other words, the slave will fire after the master had fired and so the flashes won't be synchronized?

[D_CeLiRaToR]

the signal travels at the speed of light so any "delay" is not an issue assuming you stay within the cameras max sync speed.

[PacAce]

Suppose I'm using Canon's Speedlite optical slave system, the slave will fire when it detected that the master had fired right? But won't there be a delay? In other words, the slave will fire after the master had fired and so the flashes won't be synchronized?

The command signal emitted by the master is fired before the shutter is opened. The command signal will set up the slaves to fire at the appropriate power level. Then, as soon as the shutter opens, the master sends the final "Fire!" command which then causes the slave to fire almost instantaneously thereafter so, no, there will not be a delay that's going to cause a sync problem.

[tim]

Optical slave is much, much faster than audio slave. For a while every time I hit the shutter i'd yell "now", and my assistant would manually fire the flash. To start with I needed 2s exposures to make sure the 2nd flash got into the exposure, but with practice we got that down to 500ms. When I use two slaves you have to allow a little more slack, as my 2nd assistant is a little slower than my first assistant. Adjusting the power levels takes a little time too, but it was quite easy - I just shouted "Jim, up one stop, Suzy, down one stop!". They were great for positioning though, I like voice activated light stands. I got a little hoarse from the yelling, especially when the flash was more than a few meters from the camera.

I used this setup a lot at weddings, and the guests at the reception were finding it a little distracting especially during speeches, so I switched to radio slaves - Skyports (http://www.bhphotovideo.com/c/product/505381-REG/Elinchrom_EL_19360_EL_Skyport_Universal_Radio_Slav e.html/BI/2312/KBID/3114) to be exact. I can now do 1/250th exposures, and I don't lose my voice every time I take photos! ;)

[Jamesino]

The command signal emitted by the master is fired before the shutter is opened. The command signal will set up the slaves to fire at the appropriate power level. Then, as soon as the shutter opens, the master sends the final "Fire!" command which then causes the slave to fire almost instantaneously thereafter so, no, there will not be a delay that's going to cause a sync problem.

What about the optical slave system on studio strobes? Is the command signal also fired before the shutter is opened?

[Roona99]

Suppose I'm using Canon's Speedlite optical slave system, the slave will fire when it detected that the master had fired right? But won't there be a delay? In other words, the slave will fire after the master had fired and so the flashes won't be synchronized?

The canon system uses infrared. This system sends the signal in beforehand.

There are also products that use a "flash detector" to fire the slave when ever another flash is detected. This is different and naturally there is a short delay, but how short I have no idea and I would like to hear this also..

[tim]

What about the optical slave system on studio strobes? Is the command signal also fired before the shutter is opened?

Studio slaves use a simple optical slave setup, not like Canon's complex wireless ETTL. When it sees a flash go off the studio light goes off.

The canon system uses infrared. This system sends the signal in beforehand.

There are also products that use a "flash detector" to fire the slave when ever another flash is detected. This is different and naturally there is a short delay, but how short I have no idea and I would like to hear this also..

The Canon system uses visible light. Optical slaves are generally pretty quick, fast enough for general use.

[Titus213]

As Leo said, the Canon system fires a series of commands before it fires the master and slave flash. When the shutter is fired everything will be in sync.

Very fast.

But think about regular optical slaves - they work within the shutter speed also and the trigger is the main flash, not a series of command flashes.

[PacAce]

The canon system uses infrared. This system sends the signal in beforehand.

There are also products that use a "flash detector" to fire the slave when ever another flash is detected. This is different and naturally there is a short delay, but how short I have no idea and I would like to hear this also..

Both the Canon wireless IR system and the regular optical slave triggering systems use the same band of light spectrum, both visible and IR. And both systems respond instantaneously when triggered. The difference between the two is that an ordinary optical slave trigger will fire as soon as it detects any light, IR or visible. The Canon system uses commands that are carried via the light so there's "intelligence" built into this system.

[Roona99]

The Canon system uses visible light. Optical slaves are generally pretty quick, fast enough for general use.

If The canon system doesnt use IR, then how does the off camera flash know when to fire when using a ST-E2 + 430EX for example. ?

[tim]

If The canon system doesnt use IR, then how does the off camera flash know when to fire when using a ST-E2 + 430EX for example. ?

See Leo's post - it uses both. Light is light whether we can see it or not.

[Roona99]

See Leo's post - it uses both. Light is light whether we can see it or not.

Yes, but in your earlier post you wrote visible light

IR is not visible light.

[tim]

Yes, but in your earlier post you wrote visible light

IR is not visible light.

The Canon flashes (580EX etc) use visible light, not IR, i'd just forgotten about the ST-E2. I suspect it has a flash type thingy in there with a filter that only lets part of the spectrum out.

[Roona99]

The Canon flashes (580EX etc) use visible light, not IR, i'd just forgotten about the ST-E2. I suspect it has a flash type thingy in there with a filter that only lets part of the spectrum out.

Really ? I thought you could use the 580 as a master and disable the flash tube and only use it as a transmitter.

How does it transmit the trigger signal then ?

[PacAce]

For clarification, the fact of the matter is that all Canon slave flashes only see IR light. That is the reason the system is said to be an IR wireless system. The flash emitted by the 580EX or any master flash contains both visible light and IR light and it's the IR light that gets through to the slaves. If there were no IR component in the flash light, the slaves would not work. The light source in the ST-E2 also emits visible and IR light but there's an IR panel in front of this light source so that only the IR light goes through. That is the reason you can't see the command signal emitted by the ST-E2 whereas you can see it from the 580EX. :)

[PacAce]

Really ? I thought you could use the 580 as a master and disable the flash tube and only use it as a transmitter.

How does it transmit the trigger signal then ?

Even though you can "disable" the flash on the master, it really isn't disabled. The only thing that happens when the master is "disabled" is that the main flash is prevented from firing so that it doesn't contribute any light to the picture. The master flash will still fire to emit the command flashes to the slave flashes.

[Roona99]

Ok. Thanks. Now it makes sense.