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.

  • 1
    The Canon 420EZ is a film era ATTL flash and only usable with digital cameras in Manual mode. It has no master or slave functions so please explain what you mean when you say you "set the 420-EZ to slave mode". Feb 26, 2017 at 0:50
  • Here is a link to a 430EZ manual for anyone interested in this question: cameramanuals.org/flashes_meters/speedlite_420ez.pdf Feb 26, 2017 at 0:56
  • Did you set or regulate the power of the studio strobes and if you did how did you determine what settings to use. Did you manually fire the strobes and measure their output with a light meter. What lighting ratio do you want to between the studio strobes and the camera flash. I:E: are the strokes to start a brighter on camera flash just for Fill, please explain your process in more detail
    – Alaska Man
    Feb 26, 2017 at 4:56
  • edit: that should read are the strobes supposed to be 2 stops (or ?) brighter then the on camera flash.
    – Alaska Man
    Feb 26, 2017 at 5:04
  • 1
    Possible duplicate of How to stop built-in flash from pre-flashing?
    – Michael C
    Feb 27, 2017 at 0:15

2 Answers 2


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.

  • Thanks Caleb. I think you've hit the nail on the head! Looks like I can't change the mode of this flash to manual though, so looks like a new flash would be required to use it the way I am trying to use it.
    – Wayne
    Feb 26, 2017 at 9:55
  • @Wayne Do not use a hot shoe flash to fire the other flashes, use a radio trigger. But one DIY hack you can do is point the head tho the ceiling and mask it a bit with some tape. The Pre flash will light so little the scene that a) A more powerful flash will fire. b) This pre flash do not fire the slaves, but the second one do.
    – Rafael
    Feb 26, 2017 at 15:40

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.

examples of PC terminals

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.

  • Also: You have not answered what your lighting ratio is set to be but if your on camera flash is not part of your lighting ratio and is only being used as a slave/trigger you could replace it with a radio slave that would fire the studio strobes.
    – Alaska Man
    Feb 27, 2017 at 1:52
  • @Alaskaman Was this comment intended for the original question? I haven't mentioned lighting ratios at all.
    – Michael C
    Feb 27, 2017 at 1:54
  • Yes, comment for ratios was for the OP as i had requested he give us more info on his lighting goals. i posted here as an addendum to your pc in the hot shoe advise as an alternative if he did not need E-TTL communication. sorry for the confusion.
    – Alaska Man
    Feb 27, 2017 at 4:44
  • @Michael Clark - My camera does have the PC terminal connector, and connecting the cable from the Studio flash unit to the camera also does not trigger the flash. Again, I'm not sure why this did not work either. I can only think the supplied cable is incompatible with my camera (the cable came with the Prolinca flash), or it is not making a proper connection.
    – Wayne
    Feb 27, 2017 at 9:03

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