A step-up ring connects two different filter thread sizes, from smaller to larger. It bridges two different filter thread sizes by having each dimension on each end. It's a ring that you screw onto your smaller lense filter thread size so that you can attach larger filter on the larger end. For example, a 67-77mm step up ring would connect to a lens with a 67mm filter thread size, and expose a 77mm filter thread to the outside so you could attach an Outex 77mm front glass port.
Keep in mind that as explained in our Questions/FAQ page menu, by adding physical distance between the lens and the Outex port, you may introduce vignetting into the image by using step up rings. So while it's a "cheat" way of using the same Outex port with multiple/different lenses, you can eliminate vignetting by using the correct matching Outex port filter thread size that matches the lens filter thread size.
