My focal length began changing years ago. I started shooting flintlocks in 2000. Was quick to find the rear sight just a little fuzzy although I was shooting quite well at the time. The issue progressed and I found myself getting progressive lenses. That was a big help! But I also discoved a little device called A "Merit Optical Attachment". It attaches to one's glasses with a small suction cup and the eyepiece swings out of the way when not needed. It also has a adjustable aperature. I use it when shooting from the bench. It's a slick little deal.