How does a lens back focus?

I have a 24-70 which was pin sharp when I first got it. After 3 years of use it's starting to back focus at the wide end.

My question is, if focusing in DSLR cameras is based on contrast or edge detection through the lens, how does a lens end up back focusing?

I've come to understand that the focus sensor focuses by trying to find a point where the contrast is greatest. The sensor sends commands to the focus motor to move until the best point of focus is achieved. That being the case, surely a lens that has "slipped" a little would still be given commands to move it's focus ring until the image is in focus.

My other lenses are fine, so this leads me to think that it's not my 5D's problem.

Just an odd thought, most probably my understanding of how focusing works is wrong.

(d)SLRs in live-view mode and P&S Cameras typically use contrast-detect AF, while SLRs actually use a method of focus called phase-detect.

It's typically faster than contrast detect, although functions a little differently.

http://en.wikipedia.or​g …Autofocus#Phase​_detection

When the camera focuses, the light path is actually getting split, and going to both the viewfinder, and the AF sensor.

Then the camera flips the mirror up to take a picture, and the light path goes directly back to the sensor. Since the light path changes, and AF isn't re-done, there might be some difference in focus.

The camera tells the lens to turn the focus ring/motor a certain distance and the lens moves and tells the camera, I'm done. It's the "certain distance" in the lens that is the culprit. Maybe the lens has developed some sloppiness in it's internal moving parts and there you go, misfocus.