I have to blush - I wore the first Prescription Dive Mask four years (hundreds of dives) without noticing the upper half of the lens is the prescription part, and is in fact bonded to the original flat glass of the mask. The inside of the lens is where the added glass is bonded, giving it a definite 3-D aspect, as well as a "step" where the top part of the lens stops and the original lens continues.
I don't think I ever noticed anything unusual after I put the mask on and stepped in.
The new mask (new prescription) is made the same way.
Aside from the fact the prescription portion of the mask is curved, it makes the lens about twice as thick as the original flat lens. On mine, that's approximately the top half of the lens.
I am guessing the thickness and size of the prescription lens is dictated by the strength of the prescription. The stronger the prescription, the thicker the added lens.
My distance prescription is -4.0 for comparison.
Learn something every day!
Edited by wydeangle, 09 September 2018 - 04:28 PM.