Some manufacturers will say their optic is parallax free.  When you look at the fine print, this is true only from 50 yards out.  If you aim at something closer is your eye is not in the centre, then your point of impact can change.  This can cause a problem if you are shooting at something close in like a turkey or a squirrel.  But in bullseye handgun shooting on a 50' range this can be a big problem.  Holosun technology is parallax free from 25 yards!