If you don't have a screw-in filter that's the right size for your lens, step-up and step-down rings work as size adapters.
Camera lenses come with different lens diameters at the front of the lens. Chances are, if you’ve got a couple of lenses in your camera bag, you’ll need a couple of different-sized screw-in filters to fit them.
Ideally, you’d have a set of perfectly sized filters for the diameter of each lens in your kit. But that’s not practical or cost-effective most of the time—good-quality filters aren’t cheap! Carrying three or four polarizer filters, for example, can be pricey and cumbersome.
So if you don’t have a specific filter size that you need for your lens’s diameter, a good option is to use a step-up filter. They’re also sometimes also called stepping rings or adapter rings. Not to be confused with lens adapters (or lens mount adapters), which let you put one type of lens mount on a camera body that has a different style of lens mount.
All step-up or step-down rings do is adapt the diameter of the filter to match the lens diameter. They don’t have any glass or electronics, so they have no bearing on the optical quality or features like autofocus.1
You can but them individually or in kits like this one.
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