Earlier today, in one of the closed discussion groups for photographers, one of my local friends posed a problem that he was getting random exposures when he used his flash to trigger the strobes. Somehow, he was able to set the camera manually and balance this with the strobes. The effect was as if the strobes were firing after the exposure. The strobes went off each time, but almost always missed with the exposure on the camera. Slowing the camera shutter speed down made it worse and even less predictable. Using the max sync speed on the camera, sometimes it worked but mostly it didn’t. When the flash was attached with a sync cable, it always worked and gave a correct exposure. I have experienced this myself before I had a backup transmitter to trigger my studio strobes.
Most of us use some sort of radio trigger to get around this problem. But, you never know when knowing something like this will come in handy. I personally use Pocket Wizards, and others use other radio triggers or optical triggers. After some research, the one photographer was able to find a post on Model Mayhem by Stefano Brunesci stating that the problem was caused by the pre-flash exposure light emitted. As is normally the case, their are a few of us who add our comments in this forum.
With Canon equipment, when shooting in ETTL mode, there are two flashes that occur: a pre-flash to figure exposure, and then the actual flash for exposure itself. These usually occur so closely together that you can’t notice there are two flashes of light. When in manual mode, this pre-flash shouldn’t occur, but apparently does occur. After testing this morning, the following results were discovered:
a) Setting the flash and camera in program mode with ETTL: nothing comes out and is unpredictable (expected because of the studio strobes).
b) Setting the camera and flash in manual mode: random results that work once in a while, but exposure is still mostly unpredictable because of the studio strobes not being triggered consistently.
c) Setting the camera and flash in manual mode, but using the flash exposure lock: the “pre-flash” triggers both strobes and Wein strobe flash triggers at up to 12 feet. Waiting for the strobes to recover, then tripping the shutter causes the exposure to be dead on each and every time.
The flash exposure lock on Canon cameras is the “*” symbol on the grip of the camera, and easily pushed with your thumb. When you push it, the appears to go off, but it is just the exposure calculation flash. Depending on the grade of camera you have, either you press it once and release (for pro grade) or you may have to hold the button (consumer grade). If you press and hold a pro grade camera, the exposure flash repeats repetitively.
Note on the test didn’t include any flash modifiers, specifically soft boxes or parabolic umbrellas, so I don’t know what their effect will be on this. With the Wein flash triggers, though, the results should be similar since they are attached to the studio strobes via a PC cable and can be moved to where the flash can be seen.
My next experiment is to take this to the studio and try it out with distances of 20 or more feet, and see if there is a limit to where the exposure pre-flash doesn’t interfere with triggering studio strobes via the flash sensor. As I am not in a studio for a couple of days, it will have to wait until then. I will update the end of this post with an edit once I have.
I would assume that Nikon equipment works in a similar fashion and would give the same results. I do not know which digital Nikons have a flash exposure lock, but some do and some don’t. My older film Nikon cameras do not have such an option.
Thanks to Dario Impini, Paul D. Andrea, and Dauss Miller for their comments and suggestions in the forum as this was a team effort in creating the information found in this blog.