Adapter rings

From Photography Wiki

Adapter rings are used to mount accessories like filters on lenses with the wrong filter thread size.
Two categories are available:

• step up rings to adapt a large filter on a smaller lens
• step down rings to adapt a smaller filter on a larger lens (this will most of the time cause some vignetting)

The advantage is to be able to use a single filter (for instance a 77mm polarizer) on different types of lenses.