After some 30 years of various bifocals, initially found that a separate pair of reading glasses that were stronger than the bifocal of the prescription were alot easier to tie flies to start. After getting used to the bifocal glasses with the usual tying leaders & tippet knots along with reading magazines, newspapers & books the prescription was updated. That's when a Giraffe Magnifing & Lighting system was my personal fly tying prescription for the next newest pair of bifocals as the reading glasses became not quite good enough for the little hooks in the #20's, but the eyes had finally gotten accustomed to the daily split-level viewing ........& with the magnifier tying was less of a problem