[Charlie]

Вот что пишет Canon:
Your lens can go to infinity - and beyond! Yes, many lenses can be adjusted past the infinity marking on the lens barrel.

It’s not a problem. Such lenses are intentionally designed this way to allow the lens to compensate for the changes in optical characteristics that occur when the lens is subjected to ambient temperature changes. (the focus of lenses can be affected by thermal expansion)
т.е. - это сделано специально для компенсации, например, теплового расширения.