Tried strobes as OCF for the first time...

Can someone explain to me the pre-flash issue? and how that could be contributing to underexposed images?

What does this pre-flash do? Does it fire the flash too early? (before the shutter opens / closes).. but even if it did.. wouldn't my on camera flash still fire normally?

In the pictures I took, I couldn't even see any effect of the on camera flash even though all 3 of them were firing. If the pre-flash is the issue here.. how do I switch the optical flash off and only use CST / CSXSV to fire the einsteins?

Sorry for the noob questions.. I've never used strobes previously so just a bit confused.

The preflash gathers info for the ETTL system to use, but consequently causes the optical slaves on the strobes to trigger just before the flashes are supposed to fire, thus not contributing anything to the exposure. To turn them off, just toggle to the setting on the Einstein and set them to off (read the manual).

5W0L3 wrote in post #15233291
Can someone explain to me the pre-flash issue? and how that could be contributing to underexposed images?

What does this pre-flash do? Does it fire the flash too early? (before the shutter opens / closes).. but even if it did.. wouldn't my on camera flash still fire normally?

In the pictures I took, I couldn't even see any effect of the on camera flash even though all 3 of them were firing. If the pre-flash is the issue here.. how do I switch the optical flash off and only use CST / CSXSV to fire the einsteins?

Sorry for the noob questions.. I've never used strobes previously so just a bit confused.

The simple solution is to not use ETTL in conjunction with strobes. ETTL can't take into account the output from the Einsteins, and will lead to overexposure as a result. I also am not sure if the preflash signal is transmitted through the sync port, if it is it will still trigger the flash. Just don't use ETTL. Problem solved.

The pre-flash is a series of pulses (that most people can't actually see as pulses) that the Speedlite transmits to determine what flash power to use before the exposure occurs. It's sends out some light, the camera see's how much of that is added to the scene, and makes a calculation to determine how much power will be needed during the exposure... all happening in fractions of a second. When you are using the Canon optical wireless system, the information is also sent to the slave Speedlites during this time.

Note... I don't think the sync port accounts for pre-flash. I've heard of many event photographers using this setup w/o issues.

Scatterbrained wrote in post #15233311
The simple solution is to not use ETTL in conjunction with strobes. ETTL can't take into account the output from the Einsteins, and will lead to overexposure as a result. I also am not sure if the preflash signal is transmitted through the sync port, if it is it will still trigger the flash. Just don't use ETTL. Problem solved.

I made a thread about this a week ago. ETTL with FEC will work fine if you just add a stop or two of 'ambient' light with strobes. Just need to disable the optical slave.

5W0L3 wrote in post #15233291
What does this pre-flash do? Does it fire the flash too early? (before the shutter opens / closes).. but even if it did.. wouldn't my on camera flash still fire normally?

Preflash is the flash pulse that the flash fires, while in ettl mode, before the shutter opens that the camera uses to calculate how much power the flash needs to use during the actual exposure.

The optical slave on your strobes read this preflash as the actual flash firing therefore they fire before your shutter opens.

The reason you're still getting an under exposure is because 1. Only your on camera flash is firing while your shutter is open and 2. The camera is reading the premature firing of your strobes as light in the scene so it is telling your on camera flash, that's in ETTL mode, to use far less power then it actually would need to light the scene.

Like others have said, take the on camera flash off of ETTL mode.

gremlin75 wrote in post #15234086
Preflash is the flash pulse that the flash fires, while in ettl mode, before the shutter opens that the camera uses to calculate how much power the flash needs to use during the actual exposure.

The optical slave on your strobes read this preflash as the actual flash firing therefore they fire before your shutter opens.

The reason you're still getting an under exposure is because 1. Only your on camera flash is firing while your shutter is open and 2. The camera is reading the premature firing of your strobes as light in the scene so it is telling your on camera flash, that's in ETTL mode, to use far less power then it actually would need to light the scene.

Like others have said, take the on camera flash off of ETTL mode.

... or just turn off the optical slaves

drvnbysound wrote in post #15234091
... or just turn off the optical slaves

True, but I find it easier to use all ETTL or all manual instead of trying to balance the two. This is espically true for anyone just starting out with flashes.

you can use the einsteins in conjunction with ettl on camera, however the strobes must have the optical slave turned off. I've already tested this and others have already echoed the fact that the optical slaves are being triggered by the pre flash. Test it again and you will have discovered that you won't have this issue anymore.