This is what I found to work. On the 90mm I use a 77mm to 58mm step down adapter on the front and my 58mm filters. The glass is small on the front element and the housing is the one size barrel of the RB67 design diameter. On the 180mm lens I use 49mm filters that fit inside the inner metal tube in the rear of the lens. The filter fits exactly into the diameter and doesn't block the rectangular metal cutout with rounded corners. Any filter larger won't fit into the body. There is no vignetting on either filter lens locations.