I recently obtained a Nikon D5000 via ebay. It came with a 55-200 lens and I added an 18-55 I purchased separately elsewhere. With both lenses at various times, the auto focus goes to the extreme of a completely out of focus image and stays there. I have found that the only way to "correct" the problem is to switch to manual, focus, and then swing back to auto. The lenses are both Nikon and the camera works well most of the time, but this one problem is annoying at the least. I have attempted to clean the contacts on the body but had no improvement in results. There doesn't seem to be a pattern to the problem, just random occurrences. It can happen as soon as I turn the camera on or immediately after shooting pictures where it was working fine.
#1. "RE: Auto Focus doesn't always" In response to Reply # 0
It might help if you noted what focus mode you are using. While it is possible it ia fault in the camera the mode selected may also have a clue to your problem. In that case it's an easy solution for you.
#3. "RE: Auto Focus doesn't always" In response to Reply # 2
Welcome to Nikonians!
The Mode Selector Dial on top of the camera is the "Exposure mode" dial. In Auto "exposure mode" the camera controls the exposure, AF mode, AF Area mode, etc... performing like a P&S camera. It is very difficult to find the root cause of the problem when the camera is controlling the AF mode and the AF Area mode.
Does the problem happen when you are indoors, in low light?
Does it ever happen outdoors in bright light?
When the problem occurs, does your subject have enough contrast for the AF system to work?
#4. "RE: Auto Focus doesn't always" In response to Reply # 3
The problem has occurred in direct sunlight or indoors. The lighting and contrast don't seem to make a difference. I was shooting race cars the other day, never moving from my spot, not sweeping the camera very much. The first shot worked fine and the next one, of a different car at the same point on the track went to complete blur. I needed to reset the lens setting from A to M, manually refocus, and the flip back to auto. It focused and worked fine. But it did happen again a while later, same seat on the hill similar lighting situation. It almost seems like the auto focus isn't getting a good feedback, yet the lens is clean and the connectors all appear to be clean as well.
My experiences with newer cameras is quite limited. I owned my Konica T3 for years before giving up on film and going digital. And the one I had before this one was a simple Olympus with a glorified zoom lens, but I've forgotten the model.
#6. "RE: Auto Focus doesn't always" In response to Reply # 5
Sadly I don't have any. In the auto focus mode it will not take the picture and I always just attempted to get a clear picture. However, I'll try and remember to take the shot if possible and get them posted.