After 13 years with my US Divers mask with pop-in diopters, I decided to replace the mask and get proper prescription lenses put on.
The main reason was to replace the mask with one that has a lot wider field of view. I had a revelation when doing wide angle video just how much I was NOT seeing around me with my mask. I contacted Prescription Dive Masks in California, and they sent me three different masks to try on. I picked one and sent them back to have the lenses glued in. The mask I got (XS Scuba Fusion) has side windows, and they really make a difference. Before I could have someone swimming right next to me, and I would never see them.
Next was to get a new prescription, which I had done at Costco. To my surprise, my vision had improved by about a diopter since my previous check a few years back. For some reason, after 30 years of the same prescription, my eyes were a bit better.
Next I had to mark the my pupil position on the mask. Essentially look in the mirror and put a dot on the mask where the center of your eye looks out.
Next I had to mark a line where the bifocals started. This seems a matter of experience as you don't really know what this is going to be like until you get in the water. I picked the best spot I could think of, and it was OK, but if I had to do it again I would lower the bifocals a little bit.
I sent back the marked-up mask and waited. I got the finished mask back soon before I left to dive Bonaire, and it was then that I really got experience with it. My thoughts:
1. The wide angle view was a revelation! Really appreciate it after so many years of looking through a tunnel.
2. The glass is big, and it sometimes ended up pressing on the bridge of my nose. This could be a problem, but if I position the mask carefully it (barely) doesn't press on my nose.
3. The bifocals. First, it took days or weeks to get used to the bifocal line cutting across your vision. At first your brain tells you the mask is half-full of water. Second, you can see a lot through a small area, and I could have used a bit more regular vision, a bit less bifocal. Should have lowered the bifocals about 1/8th of an inch.
But the view! My aging eyes have no problem reading any gauges. I'd say the focused view goes about 3 feet out, and it's quite easy to hover along the bottom just looking through the bifocals. Or the regular prescription part just above the bifocals. At about 3 feet you can use either.
My vision is now much better underwater than it ever was. Even when my old mask was new, my 'presciption' was always off at least 1/4 diopter, since the diopters only came in half-diopter increments.
Highly recommended - https://prescriptiondivemasks.com/