Many of us trust our autofocus implicitly – because it’s just one less thing to worry about, really. But what when the lens starts to do weird stuff? My mate Chris over at DSLR Blog has the skinny…
If you auto-focus on an object the camera will attempt to fix the focus at the correct distance between the camera and the object.
Front focusing is when this calculation goes wrong and it focuses before the object, back focusing is where it incorrectly focuses behind the object. Either way what you achieve is a photograph where the focus is in the wrong place making your object blurred or soft.
Manually focusing still works but in effect something is wrong with either the lens or the camera.
Some more info, along with tips as to how you can test for these problems, in the Front and Back Focussing Explained article.