Cataracts
Several years ago I went to the doctor with my daughter who was having lasik eye surgery. I talked to her doctor about my situation and told him that I had cataracts, but didn't know if they were bad enough to have them removed yet. He examined my eyes and the good news for me at least, was that they were bad enough to do the surgery.
I hadn't been aware that a new lens is put in the eye at the time cataracts are removed. I just thought they cleaned the original lens. I was shocked to find out that a new lens would have (hopefully) a vision of 20/20. Previously, I had the vision of 2400, and I started wearing glasses at age five.
My surgery wasn't as successful as I'd hoped. My left eye now had a vision of 20/40, which was wonderful compared to what it had been! And the right eye was 20/80. Still better, but not good.
I resorted to wearing a contact on my left eye to bring the vision to 20/20. And, left the right eye as it was - which created what's called mono-vision.
Now I could see fairly well with distance and reading.