Off camera flash problem

[Montec]

I purchased an off camera slave flash, triggered by the on camera flash.

When ever I use it the thing fires fine but all my photos that I take when it is on are completely black!

I have tried every shutter speed and f stop...any ideas what the heck is going on?

Guide # on this slave unit is 90 and 1/1000 sync speed

[cmason]

What kind of camera?

If you are using a camera with e-ttl (canon) or i-ttl (nikon), they fire 'pre-flashes' for the camera to judge the correct flash and exposure setting. Many times this 'pre flash' triggers the off camera flash, causing it to fire before the shutter opens.

You need one of these:

Wien digital slave (http://www.bhphotovideo.com/bnh/controller/home?O=productlist&A=details&Q=&sku=263025&is=REG&addedTroughType=search)

[Montec]

But why are the shots totally black? If I take the shot without the slave set to go they turn out ok...as soon as I turn the slave on I get a black picture.

[cmason]

But why are the shots totally black? If I take the shot without the slave set to go they turn out ok...as soon as I turn the slave on I get a black picture.

two possibilities: 1) the exposure is set expecting flash, and then there is none, so it is very dark, or 2) you have set a shutter speed faster than the sync speed, so the flash is firing after the shutter closes. Set your shutter speed on 1/60 or so.

[Montec]

two possibilities: 1) the exposure is set expecting flash, and then there is none, so it is very dark, or 2) you have set a shutter speed faster than the sync speed, so the flash is firing after the shutter closes. Set your shutter speed on 1/60 or so.

I have the on camera flash going off, that sets off the slave flash.

When I use the on camera flash alone the photo is fine. When I leave all settings the exact same on turn on the slave the photo is black.

I am confused!

[ziggy53]

CMason is correct. When the camera preflash goes off, the slave also fires, tricking the camera into thinking there is lots more light (it was only expecting the preflash) so it compensates by setting the aperture very small and trims the camera main flash to compensate, resulting in a black frame.

Some cameras allow you to defeat the preflash. Some slaves are "digital" meaning that they trigger on the "second" flash. You can also "detune" many slaves (partially covering the sensor)to make them less sensitive, so the slave won't sense the preflash and only triggers on the main flash.

I believe some of the Canon EX series flashes and (I think) the Sigma 500 Super flashes can be used as master flashes in a multi-flash setup, with compatible slaves, and allowing TTL operation at the same time.

I use a small hotshoe flash as the trigger and set the slaves in manual output, figuring ratios by hand (old school).

ziggy53

[Montec]

OK I think I got it.

I need to disable the pre-flash.

Thanks for the info everyone, it is appreciated