Most of the lens now come with auto focus (such as Canon's USM). I am curious to know if auto focus mechanism wear out if you use too much. Does manual focus increase the longevity of the lens? Also, is it recommended to switch to MF (manual focus) before adjusting focus? I was told as long as I don't hinder the lens's auto focusing, focus can be adjusted in AF mode. Edit: I just noticed there was a typo on the title. It should be manual without f.