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I have a Canon 5D Mark III with a Yongnuo YN600EX-RT and YN-E3-RT.

I do astrophotography and I'd like to introduce people in the foreground while firing the flash on rear curtain for freezing them. It works well with the flash on camera, but I also want to do this with a off-camera flash.

I want to try a TTL cable but since everything is very expensive (I'm in Brazil), I'd like know if someone has tried it.

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    WHY do you want to use Rear Curtain flash? As long as the people are not moving, there is no need to use rear curtain flash. Front curtain and rear curtain flash will be identical with stationary subjects. Yes, an ETTL cord will work with rear curtain flash just fine because the camera doesn’t know that the flash is attached via the cord. The camera only sees that the flash is attached to the hot shoe. Jun 21 at 22:03
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Your cheapest solution? Shoot with the flash's group set to Manual on the YN-E3-RT. Your shutter speed will also need to be between 30s and 1/30s. That should give you access to 2nd curtain sync. The user manual on page 16 says when Menu 2 is displayed, the SYNC soft button cycles you through HSS → 2nd curtain → 1st curtain.

While Canon only recently added 2nd-curtain sync over radio to their RT system with the ST-E3-RT (Vers.2), 3rd-party radio flash trigger gear from Yongnuo and Godox for Canon have had this capability for a while.

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The least expensive option after the built in flash is an ordinary flash in optical slave mode.

It won’t give you TTL, but it would not be difficult to make test shots to set the power…cheaper often means trading time for money.

The off camera flash in optical slave mode will fire when the on camera flash fires on the rear curtain.

If the exposure is long enough and you don’t want the on camera flash to fire, you can just fire the off camera flash manually…again without TTL. This would allow using pretty much any xenon flash ever made.

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