I can imagine that bifocals would cause issues. They are intended to allow you to focus at two different distances (distant objects and reading) but the Quest has a single focal length of about 1.3m if I remember correctly so at least part of the bifocal will be focussing at the wrong distance. You should obtain a set of single focus glasses or prescription headset inserts.
Yep, @user_901925786032222 is right, all of the screen area is the same focal distance so bifocals or varifocals would cause problems. I'm short sighted so I use an old pair of distance glasses in the headset... they're not quite as powerful as my current distance glasses and seem to be a little more comfortable on the eyes.
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