A filter mounting system developed in the USA and used from the 1930s to the 1970s. The filters were round pieces of glass or gelatin mounted as a rule in metal rims with no threads. The filter is inserted into the screw-in or slip-on adapter ring mounted on a lens and then held in place with threaded retaining ring. A lens hood sometimes acted as an adapter or retaining ring.
| Filter type |
Filter size
(inch — mm) |
Retaining ring size
(inch — mm) |
Lens diameter, mm |
| Series IV / 4 |
13/16 |
20.3 |
15/16 |
23.8 |
16-18 |
| Series 4.5 |
1 |
25.4 |
1 1/8 |
28.6 |
19-25 |
| Series V / 5 |
1 3/16 |
30.2 |
1 5/16 |
33.3 |
19-30 |
| Series 5.5 |
1 19/32 |
40.5 |
1 23/32 |
43.7 |
31-40 |
| Series VI / 6 |
1 5/8 |
41.3 |
1 3/4 |
44.5 |
31-42 |
| Series 6.5 |
1 7/8 |
47.6 |
2 |
50.8 |
43-48 |
| Series VII / 7 |
2 |
50.8 |
2 1/8 |
54.0 |
43-51 |
| Series 7.5 |
2 1/4 |
57.2 |
2 3/8 |
60.3 |
52-57 |
| Series VIII / 8 |
2 1/2 |
63.5 |
2 5/8 |
66.7 |
52-67 |
| Series 8.5 |
2 15/16 |
74.6 |
3 1/16 |
77.8 |
67-75 |
| Series IX / 9 |
3 1/4 |
82.6 |
3 7/16 |
87.3 |
67-85 |
| Series X / 10 |
4 1/2 |
114 |
4 5/8 |
117 |
86-114 |
| Series XI / 11 |
5 7/16 |
138 |
5 9/16 |
141 |
115-138 |