All lenses are made of pure glass that are custom made for each prescription and mask and bonded to the back of the face plate (we never alter the original seal of the masks). We can install prescription lenses in a mask you already have, or in one that we sell.
We do not "average" your prescription or use the "spherical equivalent", our lenses will be made to your exact specification on your prescripiton so the optical clarity will be the same as in regular glasses.
As with everything from UseMyFrame.com, we offer a 100% money back guarantee should you not be completely satisfied.