I'm experiencing trouble with that very issue at the moment...bi-focal lenses in which the band width is not matched to the region I look through when sighting. An additional frustration is that polycarbonate lenses, while offering the best protection, make for very soft and scratchable eyewear. Mine have saved me twice from material thrown from a lawn mower and a weed eater...therefore, worth every penny....downside is my front sight is furry.