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The built-in strobes on DSLRs are great for adding fill light in a pinch, but not for triggering the optic slaves on studio strobes. Quite simply put: modern built-in flashes are very smart are figuring out the light output, but to do so, they need to fire off an evaluative burst of light prior to image capture. This pre-flash is measured and then the shutter opens, the strobe emits enough light based on the camera settings and the pre-flash reading, and the image is captured. When the built-in flash is the only flash, this usually works just fine.
But throw a set of studio strobes with optic slaves into the mix, and it all falls apart. The built-in light sensors on the external strobes dutifully identify a burst of light and trip their circuits with the pre-flash. But then the strobe needs to recycle and store up enough of an electrical charge for the next flash of light, and this recycle period is usually longer than the time between the pre-flash and the capture moment when the shutter opens and the onboard strobe fires again. So here's the result: the only flash lighting hitting the subject comes from the onboard flash, and the big studio heads juice themselves up for the next burst of light. Which will be your pre-flash. Again. And so it goes on and on. It's tough to get around this pre-flash issue with just the built-in strobe–not impossible, but tough. (Here's how: total darkness, second curtain flash sync, and a shutter time that is longer then the recycle time for all optically-triggered strobes. This may work OK for tabletop work, but it will be very weird and creepy for live models or anything moving.)
The much easier solution lies in the center contact spot of your camera's hotshoe. This is the "non-dedicated" contact point for external strobes. The "non-dedicated" hotshoe mounted strobe gets triggered off a small electrical signal from the camera that says only one thing when the shutter button is pushed: GO!
And then the strobe fires according to whatever the settings on the strobe are set to. Some non-dedicated strobes can be adjusted to emit only a ratio of their total charge, while others have fixed output–all or nothing, in other words. A very cheap solution is this little Flashpoint strobe. But this strobe fires straight ahead only, which may lead to hotspots on the subject, and negate a little of the effect of the positioned studio strobes. Sure, you could rig a deflector in front of this little strobe to bounce the light skyward, or you could spend a few bucks more for a swivel-head unit that can bounce the light off the ceiling to trigger the external optically slaved lights. There's less chance of the shoe-mount trigger unit's output impacting the image when the light is bounced off the ceiling. (A dedicated strobe set to manual also skips the pre-flash, too, so that's another possible solution.)
But there's another much cooler way to optically and wirelessly trigger the studio strobes from the hotshoe, and it's even more economical than the bounce flash above. It's an infra red remote trigger. You see, the optical slaves on virtually all strobes are sensitive into the infra red wavelengths, so a quick burst of invisible-to-the-human-eye-and-most-off-the-shelf-DSLRs infra red "light" from the hotshoe-mounted IR flash triggers the optic slaves on the external strobes! The light on the subject comes only from the studio strobes, there's no pre-flash issues from a built-in, and no stray light from a non-dedicated visible-spectrum shoe-mount strobe to deal with whatsoever!
This is a great wireless solution for the home studio, but please bear in mind that optical triggers are considered taboo when there are multiple photographers at a location such as sporting events. In these cases, radio triggers and receivers, such as Pocket Wizards and Quantum Radio Slaves are the way to go.
Got a question for TechTock? Need tips on camera settings, or not sure what piece of gear you might need for your next big adventure? Email Me and we might answer it in an upcoming post!