Underexposed Images using Studio flash

Question

Hoping someone can help. I am attempting to use some studio flash lights in my setup.

I have a Canon EOS 1DS MKII, with a Speedlight 420EX attached to the hotshoe. When firing with just the 420EX, the photo exposes correctly. When I then turn on the 2 x Studio lights (Prolinca 250), the 420EX fires, the 2 x studio lights fire, but the image is underexposed (as if all the flashes had NOT fired).

I haven't changed any settings between firing the 420EX only (and correct exposure), to taking the 2nd, underepxosed image, with the Studio lights on.

I don't understand why the 2nd image wouldn't be correctly exposed as the Speedlight fires normally (this is what triggers the 2 x Studio lights), even if the studio lights were having no effect!

I have tried this with various shutter speeds, from 1/30 to 1/200. This has made no difference.

I have set the 420EX to slave mode and slave mode off. Again, no difference.

I have cleared all custom and personal functions on the camera.

Any help appreciated.

Answer 1

I don't understand why the 2nd image wouldn't be correctly exposed as the Speedlight fires normally (this is what triggers the 2 x Studio lights), even if the studio lights were having no effect!

Sounds like the studio lights are firing at the wrong time. Are you using ETTL mode? With ETTL, the camera fires a pre-flash to judge the exposure, and then takes the photo firing the flash at whatever power level it thinks is necessary. If the studio flashes fire in response to the pre-flash, then a) the camera is going to see a LOT more light when it's judging exposure, and then b) the studio lights probably won't fire at all during the exposure, having already fired once. That means that the camera will significantly underexpose the shot.

Unless your studio lights are compatible with Canon ETTL, you'll need to switch to manual mode and set the flash power for both the speedlight and the studio lights manually. If the lights are, in fact, ETTL-compatible, then you should make sure that they're set to the ETTL mode.

Answer 2

The camera mounted 430EX's pre-flash is a very low power pulse. The E-TTL system uses the amount of light reflected by that low power pre-flash to calculate how much of the flash's power is needed for a proper exposure.

What seems to be happening in your case is that the pre-flash is triggering your studio lights. This fools the camera into thinking the influence of the low power pre-flash is much greater than it is. So the camera probably reduces the power of the 420EX to minimum power for the main flash while the shutter is open.

The studio strobes, on the other hand, are firing before the shutter opens. Most of the energy of a studio strobe is released fairly quickly during the total duration of the flash. It seems that by the time your shutter opens the studio strobes have already released most of their energy and are either at the tail end of their flash duration or have already ended it.

For studio strobes that have the feature, setting optical slave mode to either respond to the second flash from the TTL flash or setting them to a specific delay timing so that they will flash when the shutter has just opened will solve your problem.

If your studio flashes don't have a delayed slave mode then you need to fire the studio strobes via wired connection from your camera's PC terminal.

If your camera does not have a PC terminal connector then you can use an adapter that fits on your hot shoe that passes through the full E-TTL camera to flash communications between all of your camera and flash's hot shoe connectors while only passing the center post "fire" command to your studio strobes.