Further to my post above, the following is effectively a repeat of a post I made a year or so back regarding Varifocal Lenses.
I use varifocal, photochromic glasses and they work fine but you must get the very best quality. The ones I use cost about £300 from Boots, can't remember the name but any good optician will tell you. I once tried a 'cheap' pair, only about £200 from Vision Express or one of the clones. The lenses were Kodak, I think , and they were useless.
Good, properly fitted varifocals give undistorted vision in focus at all times, without thinking about it. If you are working under your car, you may need to tilt your head back awkwardly to focus on something close and above you but in normal use it is automatic. I have no impression of anything within the frame area being out of focus. The cheap lenses were an abomination. The area which 'worked' was in a vertical strip down the middle of the lens, quite narrow too, and anything off to the side was weird. As you walked along you could see things wavering in your peripheral vision. It made me feel quite nauseous. It may take a while to get them set up properly. The worst case I had took three attempts to get it right. If the vertical positioning of the lens is not spot on, you will find that you are holding your head at an unnatural angle to get perfect vision. The best way to check this out is to go for a drive in the car. I could detect it easily on my 15 mile drive to work. If they are correctly set up, they are great. If not, not.
I used to wear bi-focals but found that I couldn't read the numbers on my radio in the Glider. Neither lens was any good. Varifocals got around that and I wouldn't go back now.
Mike W