quality control on lenses

Hi folks i'm new at this but have been reading a lot of the threads and a constant theme seems to be " i got a good copy of a lens or a bad one "
. It's making me a little nervous about investing in lenses.
So my question is how common are bad copies of lenses ?

Depends on the lens. Some are notorious for QC problems, and others seem to never "go bad". You can't really take those "bad copy" posts for gold, though...how many people with good copies do you think are out there that don't feel the need to say how good their copy is? I don't start a whole thread when I find out my lens focuses accurately. Just give your lenses a test when you get them, and if you get a problem (part malfunction, front/back focusing, etc.), just bring it back, and chances are the second one will be alright. Just don't worry...buy all the glass you want. I've not yet had a bad copy of any of my lenses, and I've had about 20 over the years.

These days it mostly seems to be a problem with focusing accuracy rather than sharpness problems.Just going by what people most post about on forums anyway (plus personal experience)
Buy from somewhere that accepts refunds/exchanges and you really can't go wrong