I don't know how it works myself, but one of my dive buddies needs reading glasses to see his computer. He went to the corner drug store and bought these little stickey reading glass lenses for a few bucks. They are a flat sticker and he afixed them inside the bottom of his mask glass where you would expect. He says it really helps him to be able to read his computer, and I am sure he also uses it to see all of the wee little critters during his dive. I will have to ask him next time we go diving together.