I have just been left some photography equipment. The item is made up of two parts: a flat square and a circular piece together with a holder and a number of adaptor rings. I am used to the circular polarizer, but am at a loss as to how this works.
It sounds like a Cokin system holder, including adapter rings for several sizes of lens threads and a couple of filters.
The Cokin system require at least three parts:
- An adapter ring that screws onto the filter threads on the front of a lens.
- A filter holder that attaches to the adapter ring. The regular holders can hold up to 3-4 filters at the same time. There are also 'slim' holders that can only hold one filter but can be used on wider angle lenses without vignetting.
- A filter. Most of the filters are square. These are a variety of types: color correction, neutral density, graduated color or neutral density (rectangular to allow for sliding in the holder to adjust the location of the gradation in the frame), and polarizers (circular with a ridged rim to allow easy rotation while in the holder.
It sounds like your square filter may be a neutral density filter.
The Cokin system comes in several sizes:
- The "A" series can fit on lenses with filter threads up to 62mm. The filters in the A system are 67mm square, 67mm in diameter (circular polarizer). The "A" system is not recommended for lenses wider than 35mm (or FF equivalent angle of view). Some people think the "A: stands for 'amateur'.
- The "P" series can fit on lenses with filter threads up to 82mm. The filters in the P system are either 84mm square or 84mm in diameter (circular polarizer). Lenses with an angle of view of up to that of a 28mm lens on a FF camera may be used with the regular "P" holder. A special single filter holder accommodates wider lenses down to 20mm (or FF equivalant angle of view). Some people think the "P" stand for 'Pro'.
- The "Z-Pro" series can fit on lenses with filter threads up to 96mm. The "Z-Pro" filters are 100mm squares (4x4 inches), 100x150mm rectangles (4x6 inches) that allow graduated color or neutral density filters to be slid in the holder in order to adjust the position of the gradation in the frame, or 100mm in diameter. In addition to Cokin filters, the Z-Pro system can accommodate 100mm/4 inch filters from other systems such as Lee, Tiffen, and Schneider.
- There's also an "X-Pro" size that is even larger. It is mainly used by medium and large format photographers as well as advanced videographers. The standard filter sizes are 170x130mm (5.3x6.6 inches).