Series filters are a type of lens accessory that was widely used from the 1930s to the 1970s, but since then has been largely replaced by the E-system of threaded filters. Series 9 filters are an exception, as they are still very common in professional motion picture cinematography. Series filters are glass (or gelatine) disks mounted in a threadless metal rim. They are attached to the lens with an adapter and a retaining ring. The filter is placed into the adapter (or in some cases the lens hood) and held in place with the retaining ring. Adapters are available in different diameters to match different front lens sizes. Some adapters are threaded, while others are designed to slip tightly on to the front of the lens or are secured there with a fastening screw. Hence, the system is highly modular and flexible. Photographers can buy a set of Series filters that corresponds to the largest diameter of their lenses and then use different adapters to use the same filters on smaller diameter lenses. Also, some retaining rings contain both male and female threads and, thus, make it possible to stack several filters.
Series filters are available as color-correction, uv protection, and sometimes also as polarizer filters. They come in several sizes. Traditionally, the different filter diameters were denoted by Roman numerals (for example, Series VI or Series VII filter), while "half-sizes" were generally described using Arabic numbering (for example, Series 5.5 or Series 8.5 filter). Over time, the Arabic naming convention also gained ground for "full-size" Series filters, so that one can find alternatively Roman or Arabic designations engraved on the filter rim.
The filter system originated in the United States, and its sizing is based on imperial measures. For size and compatibility comparisons with metric E-system filters, one should look at the retaining ring size, which is threaded, rather than the diameter of the Series filter itself. In some cases, the Series and E-systems are close and a lens hood designed for a Series VII filter might screw into a lens with a 55mm filter thread (or a Series VIII filter into a 67mm thread). However, the fit is not perfect. Also, Series filters have a slightly different thread pitch than E-filters (for example, the Series VIII filters have a 36 tpi-pitch compared to the 32 tpi for E67 filters), so that forcing a Series retaining ring deep onto a modern lens could damage the thread. Hence, it is often advisable to use commercially available step-up rings to bridge the size and pitch differences. Conversely, using E-filters with a retaining ring and adapter, for example, on older, threadless lenses, is possible if an E-filter can be found whose outer diameter matches approximately the Series filter size, so that it fits smugly into the adapter and can be secured there without "rattling".
Some of the larger Series filters remain popular for motion picture cinematography, and a few modern photographic lenses, such as the Leica Summilux-M 21mm f/1.4 ASPH, are designed for use with Series filters. Nevertheless, there are only very few filter manufacturers who retain Series filters among their product offerings. One example is B+W, which still produces Series 7, Series 8, and Series 9 filters. Yet, for smaller sizes, as well as for corresponding retaining rings and adapters to older lenses, one has to look out for available offers on the pre-owned market.
Many older Leica lenses used to take series filters. You can check on the front filter mount of different optics on the compendium-pages for Leica M-lenses and R-system optics. Some Leica R-lenses also used the series filter system for rear filters. This is notably the case for the Leica APO-Telyt line.