4-inch square ND filters: 10 f-stop ND (Lee) and 4 f-stop (Hitech). Mostly for long exposures of water. The Lee Big Stopper has a cold color cast, but that is easily neutralized in processing. Lee 4-inch square filter holder, with 77mm and 82mm adapter rings for using the above...
As I have already mentioned, there is no LCD screen, EVF, or display screen of any kind. There is also no internal ND filters or IBIS. There is also no onboard XLR inputs. Where does it sit in Panasonic’s Line Up? According to Panasonic, the LUMIX BGH1 sits above the GH series...
You do not need to use a UV/IR-cut filter of any kind (internal or external) in conjunction with L-extreme or other narrowband filters because these filters already cut 100% of UV and IR light outside of the narrow passbands of interest. An additional UV/IR-filter wouldn't do anything...