Lens Group Filters
Olympus makes 10 different types of filters: Skylight (1A, colorless), L39 (UV, colorless), ND2 (grey 2x), ND4 (grey 4x), POL (linear polarizing), C-POL (circular polarizing), Y48 (Y2=yellow), O56 (O2=orange), R60 (R1=red), A4 (81C=warm) and B4 (82C=blue).
They are available in 5 different sizes: 46mm (slide in), 49mm, 55mm, 72mm and 100mm.
The ND2, ND4, POL, C-POL are only available in 49mm and 55mm.
The 81C and 82C are only available in 46mm, 49mm and 55mm.
The circular polarizer C-POL is the successor of the discontinued linear polarizer POL for compatibility with the OM-2Sp, OM-3(Ti) and OM-4(Ti) bodies that don't indicate correct exposure with a linear polarizer.