The first number of an adapter ring is always the lens thread size, the second number is the filter size of the accessory. If the first number is bigger, it's a step-down ring What is a step up ring? Step up rings, also known as adaptor rings, allow you to use filters and accessories that are not the same size as your cameras lens. Lenses and UV CPL come in a large range of types and sizes and unfortunately they don't always fit together correctly.