Model BF 6x6-1-CLSE4 / BF 6x61CLSE4 / BF 6x61CLSEF4. 6x6 / 6.6x6.6 size. This is the 4mm thick highest quality version of this filter. Compare this glass in quality to Tiffen, Schneider, and B+W. HD Soft filters are also very useful in other areas.