Wah. U sound very well read! Lols
Btw how u know its not the focus distance?
Are you mocking me ? :bsmilie: Just kidding..
I won't say I'm well read, just that I'm a science person. Base on what TS described, if the issue is because of focusing distance or focus motor, then how can he possibly achieve focus in AF-C mode ? So I rule out the possibility that it's due to misuse of the equipment. In the last description, TS mentioned that in AF-S mode and the lens is hunting up and down to attempt focus. One possibility is there is nothing to focus such as a lens cap on or excessive dim lighting, the other is focus point is not set on something to focus on. Now the first is out because TS managed to focus in AF-C, the 2nd is unlikely because even if TS doesn't know how to use DSLR, it would at least catch something in the background or foreground to focus, instead of going back and forth.
Next is TS did mentioned he is not new to DSLR, so I'm not going to assume he is totally unaware that he need to point the focus point at the subject or too close to focus. The clue that allow me to make that conclusion is he managed to focus using AF-C and not AF-S. I have did a quick lookup on Canon 400D and it did equip with phase detection feature and seemingly coincide with the symptoms that he is describing.
Of course, I'm not totally convinced yet. Hence I ask him to check if there is anything blocking light between the lens, mirror and the top translucent thingie which is suppose to lead to the AF unit.
As a disclaimer, I don't repair cameras and I don't take them apart too. Not that wealthy to do something like this. :bsmilie:
I do like to read up on photography and its related technicalities hence I am quite informed in how camera does focusing for both phase detection and contrast detection.