Autofocus motor turns but lens does not achieve autofocus. It will try for a few seconds, then quit and generate Error 99 on the camera. On manual, lens seems to work fine.
SOURCE: Nikon D50 autofocus
I just encountered the same problem. The advice giver who forced the focus ring with autofocus engaged was right on the mark. (Thanks!) Actually, I didn't need to force it, just gave it a firm push. Voila!
SOURCE: My Canon digital rebel will not autofocus correctly.
I had this problem awhile ago - you must have accidently adjusted the focal area - it is sooooo easy to do without realising it. Here is what I was told:
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Most likely, you have changed the AF point, which only takes effect in the creative modes (Manual, Av, Tv, P, etc)
There should be a button on your camera that has a grid like pattern above it, like
-
- - - - -
-
or
so. press that, look through the viewfinder, and move the shutter or
rear wheel, you should see the different AF points like up in red, and
as you turn different ones will light up, and eventually all of them.
also, you can look on the screen on top (or back, if its a rebel) and
it will show you which ones are selected while in that mode, instead of
looking through the viewfinder.
SOURCE: 70-300DL Macro Super autofocus problem
It is likely that the lens will need parts. Sigma is not the best for repair parts. Typically, the small nylon gears that control the AF break. It is still a useful piece in manual focus. Repair costs would likely exceed its value.
SOURCE: Canon digital camera will not focus
Check your diopter adjustment. It should be a small wheel on the upper right of the viewfinder. It is designed so that people who normally wear eyeglasses can use the viewfinder without their glasses, but it can occasionally get bumped. If the diopter adjustment is way off, it is like looking through someone else's eyeglasses!
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