The first shot with the BG39 removed, shows a steady fire.
In #2, with the 403/BG39 stack, smoke is visible, but the flames appear to be nonexistent, to the point where you can see details in the blazing fire pit.
Taken seconds apart, lots of flames showing in #3, with no signs of fire in #4.
In #6, details in and through the fire pit can be seen, even though a steady blaze is shown in #5.
What I find fascinating is seeing no indication of flame or fire on the a7R's LCD screen, with the ability to see through the fire, while visually observing a blazing fire in real-time.
Sony a7R FS-conversion, Nikkor 50mm/f1.8 Ai lens, CWB with 403+BG39
#1 - 403 filter, f/1.8 for 1/400s @ ISO-400

#2 - 403+BG39 stack, f/1.8 for 1/8s @ ISO-400

#3 - 403 filter, f/1.8 for 1/640s @ ISO-400

#4 - 403+BG39 stack, f/1.8 for 1/10s @ ISO-400

#5 - 403 filter, f/1.8 for 1/500s @ ISO-400

#6 - 403+BG39 stack, f/1.8 for 1/8s @ ISO-400

Edited by GaryR, 12 April 2019 - 12:47.