Used to keep your lens cap secure on your camera strap. Unlike the bulky and vulnerable existing ones, this is slim. It doesn't need to be fully round due to my anti-rotation keying features. There are 2 anti-rotation keying features that match the Genuine Sony 40.5mm Lens Cap (ALC-F405S) that comes with the 16-50mm kit lens. These features keep the cap from rotating off the mount. These features are key, if your lens cap is not a Genuine OEM Sony, this may not work for you. As a bonus, there is a 3mm mounting hole so you can screw this to something else, like your camera bag.