Stepping rings are really useful additions to your filter collection. They are slim rings that have a male thread on one side and a female thread of a different size on the other. They are designed to allow a filter with one diameter be used on a lens with a different diameter.
You may, for example, want to use a 55mm thread diameter filter on a lens that has a 52mm thread. A stepping ring is what you need.
Stepping rings are sold in two directions – up and down. The direction is always indicated from the lens. So a step up ring allows a bigger diameter filter to be used on the lens, while a step down ring allows a smaller diameter filter to be used on the lens.
Stepping up is rarely a problem, but stepping down can cause vignetting because the filter may starts to mask the optical path, especially if the stepping increment is steep or if the lens is a wide angle. It’s always safer to step up.
The advantage of using stepping rings is that you are often able to use just one size filter on a range of lenses. especially if you plan carefully.
Say, for example, you have the following three lenses:
A wide angle with a 58mm thread, standard zoom with a 52mm thread and a telephoto lens with a 62mm thread. You could buy a set of 58mm filters to fit straight on the wide angle along with a 52-58mm stepping ring for the standard zoom and a 62-58mm stepping ring for the telephoto.
PhotographyAttic has a range of rings covering threads from 30.5mm right up to 77mm: Lens Stepping Rings for sale