I have been experimenting on these troublesome magnifications with a Mamiya 645 150mm f2.8 as the long lens and a Mamiya 645 55mm f2.8 reversed on a ring for 2.7X. Then I tried adding a Canon 1.4X III to get 3.8X . The Mamiya 80mm works well on the 150 for 1.9x and is easier because of the lower magnification. On the 6D this is all mounted on a Velbon Super Mag rail. After several trials I settled on using two 3/4 inch black paper stops in the lens stack--one very close to the back element of the 55 to block stray light and one very close to the front element of the 150 for the same purpose. The 55mm is sharpest in the stack at f4 and the 150mm is stopped down to f4 also to further block stray light. It starts to vignette at f5.6. The last refinement is the conical reflector on the end of the 55mm's back lens cap. That blocks a little more light but also reflects light from the side onto the subject. Lighting with the big lens diameter needs what help it can get.
Lights are incandescent diffused to 10 inch disks. I was having trouble with blue-white intensity with fluorescent and led sources and read some advice from Rik in another thread. Also from reading Riks advice these stacks were done with both PMax and Dmap and the Dmap image was the base for retouching mostly with PMax and the out of focus background with the frame at one end of the stack and the out of focus foreground from the other end of the stack.
A view of lens stack components:
The rig:
80 grit sand paper, full frame, 22 frame stack w/Zerene:
Lichen at 2.7X, full 6D , 63 frames stacked w/Z:
Bottom left corner at 1:1
Same lichen at 3.8X, 63 frames
Bottom right corner of 3.8X at 1:1
