"Uh — what do you mean calibrated?" Well, to put it (very) simply, when a DSLR's auto focus software isn't adjusted correctly for the specific lens you're working with, you end up with shots that are out of focus. But guess what — Jay P. Morgan of The Slanted Lens walks you through how to calibrate your lens step-by-step in this tutorial:

It might feel like kind of a rip-off to drop a ton of money on a lens that won't focus every time, but consider the alternative — dropping several more tons of money on a lens that will focus every time. According to professional camera repair technician Wes Kauffman, camera and lens manufacturers essentially build imperfect camera/lens calibration into their products on purpose. Here's why:

Manufacturing relies on producing a high quality product, not a perfect product. Could Canon and Nikon produce perfectly matched cameras and lenses? Yes, but none of us could afford them. So in order to keep cost down they manufacture equipment to fall within "Allowable Tolerances." In other words if a lens back focuses just a little, or a camera front focuses a tiny bit that is ok, because the amount of time and money it would take to get things perfect is just not worth it.

So yeah, calibrating your lens might be a pain in the neck sometimes, but it's saving you a ton of money on lens costs.

Source: The Slanted Lens