Have you ever pressed your shutter halfway to focus an image and....nothing happens? With the advent of autofocus lenses, we've become spoiled. We're so used to just pushing a button and, voila, images are in focus in the blink of an eye. That is, except when something goes awry, you keep mashing down on that shutter button yet nothing happens.
Such was the case for me this weekend when I had my 80-200mm f2.8 Nikon lens on my D300 body while shooting the NCAA Baseball Regionals. When I use the Big Dog (the 400mm f2.8) on a D3S camera body, I almost always have the 80-200mm on a second body, usually the D300 in daylight conditions because of its 1.5X magnification factor. This combo gives me the equivalent of 120-300mm on a full frame body. With a runner on second early in the Championship game, I let the Big Dog take a break and grabbed the D300/80-200mm to shoot a possible play at the plate. As is my custom, I pressed the shutter halfway to lock in on the catcher, my way of pre-focusing on an area I hope to shoot.
Nothing happened. Everything in the viewfinder was blurred. I pushed the button again. Nothing. Again. Nothing (for some reason, we humans believe that if we do something over and over again, a different result will ensue so I just kept pushing and pushing).
Finally, it dawned on me that the autofocus wasn't working. Why? I started to go through some troubleshooting. Turn camera off - turn camera on. Nothing. Take lens off - put lens on. Nothing. Switch from continuous to manual focus, back to continuous. Nothing. Rotate the focus ring. Ahhhhhh. Pressing the shutter caused the lens to focus.
Luckily, there was no play at the plate, the side was retired, and I had a chance to make sure all was well. I began trying to focus on near and far objects to test the autofocus and eventually it stopped working again. I rotated the focus ring on the lens and all was seemingly well...for a few seconds, then it was back to no focus.
Cleaning the contacts is far from rocket science. All you need are some Q-tips, some Contact Cleaner & Lubricator (Radio Shack sells it, or in a pinch you can use rubbing alcohol), and a clean work area. To clean the contacts, remove the body cap from the camera body and remove the rear lens cap from the lens. Spray some contact cleaner on the end of a Q-Tip, press on the Q-Tip end to remove excess solution, and swab the contacts. I prefer to use different Q-Tips for the lens and the camera. Q-Tips are cheap and there's no reason to use a dirty Q-Tip on the lens after you swab the camera body. After swabbing the contacts with the cleaning solution, swab them with the dry end of the Q-Tip. If you're anal like me, repeat once or twice more, or until you no longer get a dark discoloration on the end of the wet Q-Tip.
Clean the body and lens caps and replace them on their respective items, then let everything sit for 5 minutes. Attach the lens to the camera body and test to make sure all is well. Almost always, that should do the trick and everything should be back to normal. If not, the problem is likely something which will have to be addressed by a repair shop.
Hope this helps.