A single element diopter for land looses it's optical quality in Water and is therefore a waste of time. As a minimum you need two lenses with a sealed air space so you can get a magnification effect by close focus. The power of a land diopter reduces to 1/3 in water so a 10 becomes a 3.3 and rusts let alone the fact that a single element creates severe fringing problems look at some of the cheap gopro set ups to see what I mean
