A stepping ring is an adaptor that screws into the front of your lens to change the filter thread size. The idea is you can make one filter fit a number of lenses with different filter thread sizes by attaching a stepping ring in between the filter and the lens.
Their are always two numbers on a stepping ring. The filter thread size of the lens, and the filter thread size of the filter. They always start with the lens filter thread size first. You use a step up ring to attach a larger filter on to a lens with a smaller filter thread size And you step down to use a smaller filter on a lens with a larger thread.
So a 49-58mm stepping ring is a step up ring and will let you screw a 58mm filter onto a lens with a 49mm thread (see pic above). While a 67-55mm stepping ring is a step down ring and will let you screw a 55mm filter onto a lens with a 67mm thread.
Generally its better to use step up filters on wide angle lenses to avoid vignetting where the smaller filter encroaches into the edges of the frame. A step down filter is better used on telephoto lenses.
Here at PhotographyAttic we have a wide range of stepping rings from a number of brands. And we’ve reduced the price so most unbranded ones are £2.00 and ones with brand name are £3.00. Buy here: Stepping Rings for sale