Please suggest graduated neutral density filters and regular neutral density for Panasonic 20mm (46mm), Olympus 9-18 (52mm), and Panasonic 14-140 (62mm). What have you used to your satisfaction? Were you able to stack the two kinds of filters? Is there a one-size fits all filter sizes set up? I don't have experience using filters at all, but the photos with these optical filter effects are something lacking my photography. Any advice would be appreciated. Thanks in advance.