Caveat: Maybe you all know about this technique and find this thread a bit "ho hum". In which case, please ignore. But, for me, it was an exciting breakthrough that improved my success rate shooting moving subjects.
Panasonic GH4 in a Nauticam NA-GH4 housing with either the 7-14mm, 12-35mm (dome) or the PZ 14-42mm (flat port) shooting video of octopus, cuttlefish and squid in Lembeh. I wanted to be able to shoot these critters "up close" and "on the move" but still keep them in sharp focus.
I don't trust any auto-focus tracking system and have not had reliable success using it with the GH4 underwater, especially in murky conditions. I prefer to use AFL/AEL to lock in focus and exposure for the whole clip. With the camera set to AFS and a small centre focus point, I lock in the focus on the eye of the critter at the start of the clip. But, then the critter starts to move. How do I ensure that the critter's eye stays firmly in the narrow depth of field? Obviously I need to move the camera with the subject, keeping the distance between the critter's eye and the lens at the same distance. But that's quite difficult, especially checking the focus with the small monitor on the back of the GH4.
Solution 1 (attach a better monitor):
Use a large monitor that has built-in focus peaking like the Shogun. But, Nauticam have not yet released the Shogun housing. And I need to get the footage now.
Solution 2 (flip on the GH4's focus peaking):
Unfortunately, the GH4's focus peaking only works when the camera is set to manual focus (Panasonic: could this be fixed in a firmware update, please). The lens / port combinations that I'm using don't have manual focus.
But (and here is my big breakthrough), the NA-GH4 housing has a MF-AFC-AFS switch in a convenient location.
Here's what worked: Before diving, turn on focus peaking in the camera's menu and save it to your favourite UW custom setting. Underwater, set the MF-AFC-AFS switch to AFS and use the rear lever to set AFL to get a sharp lock on the subject's eye. Flip the MF-AFC-AFS switch to MF. The lens stays at the same focus distance as locked in. But now the subject is also covered with focus peaking dots. As the subject moves, move the camera with the subject, ensuring that the focus peaking dots continue to flicker all around the subject's eyes.
If it's not moving too fast (e.g. an octopus walking), I was able to keep the subject firmly in the plane of focus (9 times out of 10)...