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Old 20th April 2018
Walti Walti is offline
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Control epiphany!

I seem to have stumbled across why I have been struggling so much with the off camera TTL control of flash systems....

I "think" this is how it all works, can someone correct me if I still haven't understood it all?

I was using the FL600R atop the camera and the TTL double flash was very apparent.

When using the baby commander flash "off" you only get the single pre-flash - and you don't appear to get a double flash (no pre-flash) from the remote devices, which is all well and Dandy if you are using them as direct lighting sources....

If you are using brolly or diffusers all the calculations go to pot so the TTL fails to give a reasonable result.

If I were to switch to manual, how do you do the flash metering? take a photo and try and work out where the exposure is and try and get there by iteration?
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Old 20th April 2018
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Graham_of_Rainham Graham_of_Rainham is offline
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Lightbulb Re: Control epiphany!

First of all the best way to get consistent results is with a flash meter, but that will require you work in Manual Mode and to decide what exposure you want on the subject and then set the guns to deliver the amount of light required to produce it.

With TTL the first flash is in effect a "test shot" that the cameras exposure system uses to determine the power to fire the guns at to achieve the exposure you have set. If for example your settings are 1/250s @ F/5.6 ISO 400, and the camera determines that the test firing is under exposed the actual second flash will be brighter to a level that provides the correct exposure, and vice versa.

One way to see this working in practice is to set up a fan in a very dark room, and use two cameras. One is set to Time exposure and the other set to take a picture by firing the flash. Both pre-flash and main exposure flash is then captured by the camera set to time exposure and as the fan blades are rotating, different exposures will be visible from the pre-flash and the main flash.

Without a flash meter, then you can get a reasonable idea of exposure by using Guide Number charts (It's how we did it in the old days... )
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Old 20th April 2018
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Re: Control epiphany!

Graham, pretty much as I thought, but it appears you don’t get the pre flash from remote flash guns (even though they’re set to TTL in the flash menu) is this what you have seen or am I simply not seeing it?

When a flash is direct connected there’s definitely the pre flash, but when remote it doesn’t seem to happen!

I’ll set it all up and have another go.
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Old 23rd April 2018
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Re: Control epiphany!

Used the flash system again over the weekend, used an on camera and an off camera system.

On camera the TTL works really well and all the photos were exposed properly no issues!

Off camera I switched to manual control of the flash guns via the camera menu and got consistent reliable results. In comparison to my previous attempts I was careful about camera and flash positioning, so there were no mis-fires of the flash guns, seem to have understood it all at last.

I didn't try anything clever, but the off camera flash system did not give a discernible pre-flash from the remote flash guns, the on-camera system definitely did.

So for remote flash (with anything other than direct full flash use, e.g. reflectors, umbrellas, soft boxes etc..) I'd recommend the abandonment of TTL, even though the flash and camera manuals tell you it works!
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