Note about preorder items - Order now online to reserve yours today.  While some items in certain sizes aren't normally stocked, they can be purchased in advance for the soonest possible delivery from the factory.
 
B+W adapter rings for fitting filters, auxiliary or conversion lenses, and other attachments to lenses with different filter thread sizes. They are generally used to adapt filters or attachments with a larger diameter to lenses with a smaller diameter. 
 
The first number is the filter size and the second number is the lens size.  For photographic equipment that includes lenses with 52 mm and 49 mm filter threads, for example, they enable 52 mm filters to be used on both groups of lenses. This not only saves money, but also saves space and weight in your camera bag.
 
Features
wide range | can adapt filters and supplementary lenses to different lens diameters | saves purchase of several lenses with same filter type | flexible | can be combined if necessary (but: an increased thickness by using several mounts can lead to vignetting)

Tip:
When buying filters, first check the largest diameter required.