+4 Diopter Close-Up Lens with 55mm Thread

The minimum focus of your lens must be 12” (30.5 cm) or less to focus behind a dome port underwater. Lenses with minimum focus distances of greater than 12” (30.5 cm) will produce 100% blurry images if used behind a dome port underwater without a diopter attached. Adding a +4 diopter reduces the minimum focus distance of a lens allowing it to focus behind a dome port underwater. 

For lenses with a 55mm threaded filter ring.

Read more about Why your lens may need a +4 Diopter underwater.