The Hoya 77-67mm Step Down Ring enables a filter with a smaller attachment size to be used with a lens with a larger attachment size. For example, a 52-49mm Step-Down ring would allow you to use 49mm filters on lenses with a 52mm front filter thread size. Care must be taken using Step-Down rings in order to avoid vignetting.
Note:
Hoya also produces Step-Up Rings which allow larger sized filters to be used on lenses with smaller front filter thread sizes. For example, a 49-52mm Step-Up Ring would allow a 52mm filter to be used on a lens with a front filter thread size of 49mm.