I've got an event coming up where I need to use a couple of off camera 430EX II flashes fired with Cyber Sync triggers. The setup also will require second curtain synch, which I've never used before. So I've been sitting here going through my 5D3 menus trying to set second curtain synch and just discovered that the only way to do that is to have a powered-on Canon flash mounted on the camera. If you don't have a Canon flash attached and turned on, you can't access the menu to set the shutter synchronization.
Now, I just confirmed that if I attach the flash, set the shutter synch, then dismount the flash and hook it up to my Cyber Sync trigger, the second curtain sync that was previously set will work as expected. I have all camera settings on manual and and as as test, set the shutter to 6 seconds and the flash did indeed fire just before the shutter closed. Then I remounted the flash on the camera, entered the menus and set it back to first curtain synch, dismounted the flash back to a off-camera manual setup, and this time it fired when the shutter opened.
So there is a way to get there. But this obviously is not workable if you need to toggle between first and second curtain sync when using a manual off-camera flash setup, which I may need to do in this situation.
Am I missing something? Is there a way to set the flash curtain synch without having the flash physically connected to the camera and powered on?
You have just won the price for best QA
Seriously...the "bug" you have found is a design flaw in the Menu.
I am over 25 years in software development industry and like you recognize easily a design flaw.
Some menu option that does NOT require the flash to be attached should be available when the flash is NOT attached.
And, not - as it is now - be a hidden menu option that only becomes available when a flash IS attached.