No, both the EOS R5 and EOS R6 write in the User Manual that flash photography is not possible in FB and I was using the MT26 Speedlite.
I tried to place the body of the spiders parallel to the plane of the sensor to get as much in focus as was possible. Both spiders were very cooperative.

I have used the Helicon FB Tube
Cheers
Dennis
I did a test shot and managed to get a 5 shot stack with my flash heads at 1/128. I've not tried it in the wild yet but I'm looking forward to it.











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R5 , 600mm. F4 L IS II , RF 100-500, EF 100 F2.8 L IS macro, 1.4 Tc.III . 



