Slave flash exposure

[JeffOwen]

I have started to use a slave flash with my Canon 80D and can't understand why the 'with slave' exposure is so poor. 

The top photo shows the camera set to Manual 1/100th, f8, ISO400 and using the camera built in flash. The histogram is not perfect but acceptable.

The bottom photo uses a slave flash (this is an old Olympus with a photo cell trigger, see photo) placed about the same distance from the subject but at 45 deg. The camera settings are identical, but the resulting photo is well under exposed (see hystogram).  How can that be?

I have taken several photos and all result in the same effect.

[Ken Katz]

Your Canon's through the lens flash exposure system uses a small pre-flash to measure exposure just prior to setting off the main flash.  This is almost instantaneous and unlikely you would have noticed the pre-flash just prior to your mirror flipping up.  So what is happening, is that when your built in flash triggers the pre-flash, the slave flash sees this and triggers it's main flash.  The camera see's all kinds of light available during the pre flash and tells itself that the built in flash will need very little light to obtain a good exposure.  When the camera's main flash goes off (a fraction of a second latter), the slave flash has already gone off and is now recycling and does not have enough power to trigger again.

All camera TTL flash system converted to the pre flash methodology for digital cameras.  With film cameras, the TTL system could read light reflecting off of the film during exposure, and command the flash system to shut down when enough light has been measured.  Did not work for digital sensors.  I would use your external flash on manual (or set your aperture manually), and use the histogram to measure exposure and adjust the aperture as needed.

[JeffOwen]

Wow, thanks for that answer, it has really put my mind at rest. I will now go and try a workaround to see if I can sort out my idea of using two flash guns for a certain set up.

Jeff

[JeffOwen]

Just bought a Canon Speedlite flash and if I put it into Manual it does not send out the pre flash and my slave flash now fires in sync. Happy days!

Jeff

[rodeo_joe1]

Another option is to buy a slave trigger with an 'S2' option. This fires the slave flash with a second flash and ignores the first TTL pre-flash burst. That type of slave used to be common, but they seem to have fallen out of favour, and they probably cost nearly as much as a cheap YongNuo flash with S2 slave option built-in. 

Or just set your popup camera flash to manual. That will stop the TTL pre-flashes and fire the slave in synch. 

BTW, turning on redeye reduction will also fool your slave flash.

IMO you're better off setting all your flashes to manual for off-camera setups anyway. That way you have complete control over lighting ratios. Because partial TTL control can be a bit unpredictable to say the least.