When you want to use a filter on a lens with a smaller filter thread size, you want a step-up adapter ring.
A step-up filter adapter ring is a simple glass-less ring with male threads (for your lens) and larger size female threads (for your filter). The step up ring screws onto your lens and the filter screws onto the step up ring. It is as simple as that.
Why would you want one of these adapters? If you cannot afford a needed filter for all of your lens filter sizes, buy the filter in the largest size you need and then purchase relatively inexpensive step-up adapter rings for the rest of the thread sizes you need the filter for.
Step down rings are also available, but they seldom make sense. Unless you have unused image circle available to be blocked with the step down ring or unless the filter threads are far enough outside of the lens' angle of view to accommodate the ring, you will experience mechanical vignetting in your images. APS-C format DSLR camera body owners may be able to use a small amount of step down without a problem, but you will not want to rely on this use without testing it first.
A potential downside of step-up adapters is that they may interfere with lens hoods. If the hood is rather narrow and the step-up ring is rather wide, they may not be compatible for simultaneous use. Simply leave the hood off and shade the lens with you hat, hand, etc. when necessary.
While I still prefer to have a filter in each thread size I need (and sometimes have more than one of them for simultaneous use), I only keep CP and ND filters in my most-commonly used sizes. I have step-up filter adapter rings to mount these filters on any other thread size I need. I am once again advising the purchase of a B+W brand for this adapter.
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