When it comes to camera lens filters, there are two main types of rings that are used to attach them onto the lens: a step-up ring and a step-down ring. Both have their own purpose and function, so it’s important to know the difference between the two before using either one.
A step-up ring is typically used when you want to use a filter that is bigger than your lens’ thread size. For example, if your lens has a 58mm thread size but you want to use a 72mm filter, then you would need a step-up ring with a 58-72mm size. Step-up rings are also useful for attaching multiple filters together – for instance, if you wanted to stack a polarizing filter with an ND filter, you would need two separate step-up rings (one for each filter) in order to do so.
A step-down ring, on the other hand, is used when you want to use a smaller filter than your lens’ thread size. So, if your lens has a 72mm thread size but you only have 58mm filters, then you would need a 58-72mm sized step-down ring in order to attach the smaller filter onto your larger lens. Step-down rings can also be helpful for using screw-in type filters (like ND filters) with lenses that don’t have threads for them – by adding a step-down ring of appropriate size onto the lens, it essentially creates threads where there weren’t any before and allows you to screw in the desired filter.