[Steve]

The advice already given above is spot on. Buy locally or from a well respected internet retailer that has a good returns policy. Try before you buy and view the results or test as soon as it arrives.

In the event of you not being happy, you can either return it to the shop, e-tailer or if you are not to impatient send it to Canon directly who will test and calibrate the lens. If they do find it to be slightly out you can be assured that once it is calibrated (by hand and therefore usually much more accurately than a production line setting) you will have an excellent example.

In general Canon tend to these jobs quite quickly and will usually turn around the lens in under two weeks.

Finally, I would suspect that in the majority of cases you will be more than happy with the lens quality as standard and you shouldn’t really concern yourself with the ‘what if..’ scenario. It is not a bad thing to minimise the risk and know your options should it occur but its certainly not work much more than a passing thought.

HTH