I'm using two cameras: Canon EOS 400D (Digital Rebel XTi) and Canon EOS 450D (EOS Rebel XSi). 450D was more heavily used than 400D.
Not so long ago I noticed that 450D's autofocus system works not perfectly. I've decided to test both the cameras using a tripod, paper with some text on it and both the cameras were angled to 45 degrees to a paper. Lenses I've used are Canon 50/1.8 II and Canon 50/1.4. Both lenses have shown front-focus on 450D and nearly perfect focus on 400D.
Then I switched to manual mode. Here's how I made the test: I've turned the focusing ring to the closest position and then, half-pressing the shooting button, was slooowly turning the focusing ring, watching to the green AF-dot in a viewfinder.
On 400D, as it should, the green dot started to be visible on some focusing position and became not visible on some other focusing position. The other behaviour was on 450D: when I was close to the correct focus position, the green point started to be visible and then not be visible and then again be visible and not and then and then...
What things may be the reason of such a behaviour of the AF (or AF-confirm) system?
I think this is not a simple camera front-focus problem. It seems to be that some kind of dust could get onto a focus detector. Can my supposition may be right?